CERATOSTOMELLA RADICICOLA, the perfect stage of Chalaropsis, is a heterothallic pyrenomycete with long-necked perithecia and hyaline single-celled ascospores. Asexual reproduction is accomplished by means of two types of conidia, the microconidia (fig. 6) produced endogenously, and the macroconidia (fig. 2, 3) borne on sympodially branched macroconidiophores. Previous studies (Bliss, 1941; El-Ani et al., 1957) have revealed the occurrence of two distinct strains on roots of the date palm, the black strain (fig. 1) producing dark macroconidia (fig. 3) and black perithecia (fig. 5b), and the brown strain (fig. 1) in which both macroconidia (fig. 2) and perithecia (fig. 5a) are brown. The ascospores are uninucleate (fig. 7), and single-ascospore cultures are hermaphroditic, selfsterile, and of two compatibility types (A and a), which must be brought together before perithecia can be produced. When black and brown strains are mated, the progeny has been found to consist of black and brown thalli in a 1:1 ratio. Thus the two strains differ genotypically from each other by black to brown has been observed in vitro. Recently two mutants (fig. 1), gray and tan, which arose spontaneously in vitro were used with the other strains for further genetic investigation, the results of which are described in the present paper. MATERIALS AND METHODS. Cultures of C. radicicola derived from single ascospores were grown on two media, potato-dextrose-yeast agar (2g. yeast extract/I.) on which rapid vegetative growvth at temperatures of 27-30?C. ensures morphological distinction of each strain; and Bacto prune agar (48 g./l.) used mainly for perithecial production. In crosses between two thalli, conidia suspended in distilled sterile water were transferred from one thallus to another of different compatibility. Perithecia with mature ascospores were formed within four days. Single-ascospore cultures grown on the potato-dextrose-yeast agar medium were assorted according to their morphological characters within a week. In order to determine the mating type and perithecial color of each single-ascospore culture, a small conidial inoculum was transferred from the unknown thallus to each of two tester thalli of the same perithecial color but differing in compatibility factors. Tester thalli with perithecial color presumably different from that of the tested culture were chosen. For example, if the single-ascospore a single gene. As yet, however, no mutation from