ABSTRACTSphaeronaemella helvellaeisolates were tested for mating type and the effect of substrate on compatibility and fecundity of compatible crosses. Single ascospore isolates were crossed in all combinations and their interactions analyzed. Crosses of single ascospore isolates, either in axenic culture or in association with host fungi, resulted in erratic production of perithecia. Crosses of single ascospore cultures produced perithecia in 50% of cultures when incubated on corn meal agar amended with aXanthomonassp. The presence of the bacterium or its biologically sterile filtrate inhibited mycelial growth, reduced the time for perithecial initiation, and increased the quantity of perithecia produced byS. helvellae.Perithecial production inS. helvellae, a heterothallic mycopathogen, was conditioned by a single locus. In both field population and laboratory progenies ofS. helvellae, one allele of the incompatibility locus was more frequent than the other.