ACCESSORY, supernumerary, or B-chromosomes are present in numerous species of plants and animals. Usually individuals possessing them exhibit little physiological evidence of their presence. If the B-chromosomes alter the normal phenotype, the effect is most commonly unfavorable to the host (BATTAGLIA 1964). These chromosomes, which are usually smaller than the normal “A” chromosomes often exhibit a numerical variability in different cells, tissues, individuals, or populations due to abnormal behavior at mitosis and/or meiosis. In many species of plants the number of B-chromosomes increases in the progeny ( BATTAGLIA 1964). RANDOLPH (1941), LONGLEY (1927), ROMAN (1947, 1948), and BLACKWOOD (1956) have described the behavior of the R-chromosomes in maize throughout the life cycle and noted that the progenies of individuals with B-chromosomes tend to have higher numbers of B’s than the parents. ROMAN’S studies showed that nondisjunction of the B-chromosomes at the second pollen grain mitosis could result in one sperm nucleus containing no B’s while the other might have two or more B-chromosomes. This, followed by preferential fertilization of the egg by the hyperploid sperm nucleus would yield offspring with increased numbers of B-chromosomes as observed by ROMAN (1948a). The accumulation of B’s in maize brought about by nondisjunction and preferential fertilization is compensated in nature by the decreased fertility and vigor of plants possessing large numbers of B’s (RANDOLPH 1941). The two opposing mechanisms interact to maintain B-chromosomes at a relatively low level in the maize population. This study constitutes an investigation of the relationship between B-chromosomes and crossing over in the standard or A-chromosomes of maize and is an extension of preliminary reports ( HANSON 1961,1962). MATERIALS AND METHODS Two sublines of maize were extracted by RHOADES (personal communication) from a highly inbred strain of Black Mexican sweet corn originally produced by E. W. LINDSTROM. The two sublines, one carrying accessory chromosomes and the other with no B’s, have been maintained separately for many years and apparently differ only by the presence or absence of B-chrome somes. These two strains were used as male parents in crosses with chromosome tester stocks. Root tips from F, hybrids were prepared by the method of RANDOLPH (1940) and the number of B-chromosomes possessed by each plant ascertained. The hybrids were then backcrossed to their respective testers and the crossover frequencies studied. Two segments of the maize genome ’ Supported in part by Public Health Service training grant 2g82(c2), a National Science Foundation Graduate Fellowship, and by a Floyd Memorial Fellowship from Indiana University.