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The effects of premating light regimes on sexual reproduction and the production of spherical cells in Chloromonas sp.-D, a unicellular green snow alga, were studied using cross-mating strains 582C and 582D isolated from snowpacks associated with mixed hardwood-softwood forests in Whetstone Gulf State Park, Tughill Plateau, NY. Two preacclimation regimes were used, Vita-Lite as controls (530-700 nm peak) and blue light as experimentals (430-460 nm peak) prior to the mating experiments. In blue light, an increase in the number of matings and spherical cells (spheres) produced in the life cycle was observed as the photoperiod increased. This implies that longer photoperiods of blue light are more optimal for sexual reproduction in Chloromonas sp.-D than shorter ones. Under Vita-Lite, there was a significant increase in the number of matings and spheres with the extended 20 : 4 photoperiod compared with the shorter 14 : 10 photoperiod. Under blue light, significantly more matings and spheres occurred than under Vita-Lite using the same irradiance level of 95 μmol photons m -2 s -1 (photosynthetically active radiation [PAR] of 400-700 nm) for the 14: 10 and 20: 4 photoperiods. The results of these experiments suggest that Chloromonas sp.-D. known only from the Tughill Plateau, NY, is not reproducing optimally at this site where it grows and reproduces under an approximate 14: 10 photoperiod in early April. However, in the upper 10 cm of snow in the Tughill Plateau, a blue light irradiance level of 95 μmol photons m 2 s -1 occurs, which is optimal for this species. When these conditions are combined with a 14: 10 photoperiod, the Tughill Plateau appears to be sub-optimal for mating and production of spherical cells. Since Chloromonas sp.-D does not appear to have a dependence on a dark cycle, this would allow it to expand its geographical distribution. It may reproduce more optimally under blue light (95 μmol photons m -2 s -1 ) with an extended photoperiod (>20:4 h, light:dark) in high latitude field sites such as Lake Bienville, Quebec, in eastern North America where other species of Chloromonas are known in snow associated with coniferous forests.