To address this problem I have compiled 38 yr. of survey data on the diversity, distribution, abundance, and habitat association of mayflies in New England and Atlantic Canada. From these records the regional species pool is estimated (~216 species). Rare/Little [R/L] known assemblage is estimated (~77 species, 36% of regional pool). Habitat types associated with R/L taxa that could represent critical core habitats for the maintenance regional and subregional diversity are presented. Geopolitical subregion diversity is summarized and updated – CT, NB, NH, and ME have highest subregional diversity. Unexplained variation indicates potentially new species in 6 families. All current evidence underscores the complex and dynamic nature of the regional species pool, which includes the possible range expansion of Parameletus midas.