Sordaria brevicollis is a filamentous ascomycete previously considered to be exclusively heterothallic. Here we show that S. brevicollis can exhibit homothallism, and describe the genetic and environmental factors favouring the process. Both mating types can produce perithecia in unmated cultures, but only in the uncrossed perithecia of mtA are ascospores formed (in <30% of the fruitbodies). In most cases, only one or two asci are produced in any uncrossed mtA perithecium, although as many as 13 have been observed. The ascospores show no segregation for mating type; all are mtA. Heterokaryon analysis established that meiosis and recombination occurred during ascospore production in both homothallic and heterothallic development. Homothallic fruiting is inhibited both by crossing with the opposite mating type, and by light.