The mayfly Baetis tricaudatus is an abundant, widespread, and ecologically important multivoltine benthic macroinvertebrate that is found throughout most of North America. Baetis tricaudatus belongs to the Baetis rhodani species group, which is known to have cryptic species. Some investigators have found that B. tricaudatus morphospecies have cytochrome oxidase I (COI) diversity >20%. However, no investigators have examined whether this diversity is structured temporally, with some haplotypes being more common in certain years or seasons than others. We sequenced COI from 371 B. rhodani specimens. The 371 rhodani species group sequences generated fell into 2 well-supported clades, one with 38 Baetis adonis specimens and another with 333 B. tricaudatus specimens, which were the focus of our study. We examined the temporal and spatial dynamics of genetic diversity in B. tricaudatus populations from northern California using COI haplotype networks. The maximum genetic diversity among B. tricaudatus specimens was 1.7% and was found at a single site (Austin Creek). The same 2 dominant haplotypes of B. tricaudatus were consistent through years, sites, and seasons, and Φ_ST values were correspondingly low. In 2 intensive sampling events, each with >40 individuals examined, intrapopulational divergence was 1.2 to 1.4%. This result suggests that most of the genetic diversity for this species in this system could be captured in 1 high-effort sampling event rather than in smaller, long-term monitoring events. Our results suggest that, based on the sites examined, Russian River populations of B. tricaudatus constitute a single species with no evidence of cryptic diversity.