The DNA sequences of 42 spontaneous mutations of the mtr gene in Neurospora crassa have been determined. The mutantws ere selected among sexual spores to represent mutations arising in the sexual cycle. Three sexualcycle-specific mutational classes are described: hotspot mutants, spontaneous repeatinduced point mutation (RIPS) and mutations occurring during a mutagenic phase of the sexual cycle. Together, these three sexual-cycle-specific mutational classes account fo5r0 % of the mutations in the sexual-cycle mutational spectrum. One thiordf all mutations occurred at onoef two mutational hotspots that predominantly produced tandem duplications of varying lengths with short repeats at their endpoints. Neither of the two hotspots are present in the vegetative spectrum, suggesting that sexualcycle-specific mutational pathways are responsible for their presence in the spectrum. One mutant was observed that appeared to have been RIPed precociously. The usual prerequisite for RIP, a duplication of the affected region, was not present in the parent stocks and was not detected in this mutant. Finally, there is a phase early in the premeiotic sexual cycle that is overrepresented in the generation of mutations. This “peak” appeartso represent a phase during which the mutation rate rises significantly. This phase produces a disproportionally high fraction of frame shift mutations (3/6). In divisions subsequent to this, the mutation rate appears to be constant.