We examined seasonal and spatial distribution of all life stages of the diminutive cyclopoid copepod Tropocyclops prasinus mexicanus in 2 years in the nearly thermally constant environment of Montezuma Well, Arizona, USA. Although annual temperatures remained relatively constant (21 ± 4°C), densities of T. prasinus mexicanus displayed a bimodal pattern, with highest densities during the summer and winter. Also, the vertical water column was nearly homeothermal, but all life stages avoided the top 2 m of the water column during clear summer days and utilized this stratum during the winter when light was reduced. Females within the population were egg-bearing throughout the entire year, which led to high annual mean densities (>200 animals/L). Mean annual biomass of T. prasinus mexicanus was estimated at ≥50 µg/L.