1. Eight hundred progenies derived from single panicles of Poa pratensis collected in twenty-seven states of the United States were studied in the seasons 1941-1944 at Madison, Wisconsin. These included 541 Wisconsin collections. All were grown in spaced nursery rows. 2. Progenies from the United States were classified according to the geographic region from which the parent seed was collected. Observations were obtained for percentage of aberrant plants, early vigor, growth habit, aggressiveness, leafiness, maturity, late vigor, plant height, and reactions to powdery mildew (Erysiphe graminis), leaf rust (Puccinia poae-sudeticae), and leaf spot (Helminthos porium vagans). Progenies from Wisconsin sources were grouped according to soil area, length of the growing season, and geographic area where parental seed was collected. 3. Among all progenies, 75.8% included some aberrant plants. The average percentage of segregation based upon the total progeny number was 16.5. When only segregating progenies were included, this value was 20.5%. 4. Differences among regions of the United States in percentage of segregation were generally large. No close association of such differences with regional, soil, or climatological variations could be established, although interior or continental regions showed generally lower percentages of aberrant types. 5. Collections grouped by regions of the United States were generally similar to one another in variation of early vigor, growth habit, and aggressiveness. Leafiness, late vigor, and relative maturity varied more extensively among regional groups. Lots from the East Central region were generally less aggressive; those from the Northeastern and North Pacific areas were more leafy; collections from the Northern Plains and the Northern Great Basin were of earlier maturity; and progenies from the Northern Plains and Central Plains were relatively more vigorous. 6. Differences among progenies derived from the various soil areas in Wisconsin were generally small. When grouped according to the length of the growing season in the locality of collection, significant differences in percentages of segregation were found, with the 140-150-day class having a high value. Strains from areas having longer growing seasons were generally somewhat higher in percentages of aberrants than those from areas of shorter seasons. The comparison by geographical regions indicated that the Eastern Ridge and Lowlands area and the Western Upland area of Wisconsin were significantly higher in percentages of aberrants than were other areas. 7. Classifications of Wisconsin collections for morphological characters showed a generally great diversity for behavior among lots from different soil areas. Frequency distributions were similar for early vigor and leafiness for the different soils and were somewhat less so for growth habit; those for aggressiveness, maturity, and late vigor were highly dissimilar. Based upon average length of the growing season, distributions for the several characters were generally comparable with those for the soil classification. Frequency distributions for early vigor, growth habit, and leafiness were most similar. The classification for maturity showed least relation to length of growing season. Grouping on the basis of geographical areas resulted in frequencies relatively similar to those for soil and growing seasons. Classification by plant height suggested no particular relationships to region of origin in the United States. In the Wisconsin collections analysis by soil and geographic regions did not indicate prominent differences in relative frequencies of plants with respect to height. When grouped by growing seasons, the 100-110- and 130-140-day lots exhibited relatively lower frequencies of intermediate types, though this was of questionable significance. 8. Collections from the United States varied irregularly in frequency distributions for reaction to powdery mildew. Lots from different regions appeared to differ markedly in proportionate frequencies of resistant and susceptible types. Approximately 34% of the progenies were observed to be mildew-free, whereas 12% were very susceptible. Reaction to leaf rust was generally similar among groups of strains from the various regions of the United States. Of the progenies, 14% were classified as rust-free and 66% as resistant. 9. Classification of Wisconsin collections with respect to soil of origin showed certain prominent differences to exist among soil areas in the character of the distributions among disease-reaction classes. In general, however, distribution for the three kinds of diseases were similar. Considering reaction to leaf rust, collections from certain heavier soil areas were relatively more susceptible to disease than those from others. When classified according to length of the growing season, little relation was shown between length of season and relative frequencies of reaction types, though occasional exceptions occurred. No consistent relation between length of growing season and relative disease resistance was found. Collections grouped according to geographic regions exhibited no significant differences in reaction to diseases.