For an estuarine population of the ctenophoreMnemiopsis leidyi (A. Agassiz), density of small individuals, indicative of recent spawning, and total biomass were measured on 32 dates over 20 months. These observations were related to the environmental parameters of temperature, salinity, daylight, and presence of a predator. M. leidyi was present on all sampling dates, biomass was greatest in spring and fall, and ranged from 0.2 to 65 ml/m3. Occurrence of the medusae ofChrysaora quinquecirrha corresponded with a sharp reduction in ctenophore biomass in the spring, andC. quinquecirrha was shown to be a voracious feeder onM. leidyi. The number of small individuals exceeded 2/m3 on 17 of 32 sampling dates and peaked at approximately the same time as total biomass. M. leidyi occurred at salinities from 3.4 to 20 ppt (the highest salinity sampled), and the salinity where the greatest total biomass and the greatest number of small individuals occurred was lower in summer than winter. On calm days all sizes ofM. leidyi were most abundant at the surface, large individuals showed no depth preference at night, and small individuals showed a weak affinity for the bottom during the day and a strong affinity for the bottom at night. Adaptations that probably contribute to the success ofM. leidyi in an estuary are: 1) broad temperature and salinity tolerances permit the species to inhabit the estuary year around and exploit the low salinity areas inhospitable to most marine and freshwater animals; 2) ability to spawn over a wide range of temperature and salinity enables the species to exploit favorable conditions when they arise without having to depend on immigration to repopulate an area; 3) preference of small individuals for the bottom keeps them from being flushed out of the estuary; and 4) sounding by all sizes in rough seas helps them to escape the destructive effect of wave turbulence.