Egg production and hatching success of four Acartia species from the Solent-Southampton Water estuarine system were determined under different temperature (5–20°C) and salinity (15–35 psu) laboratory regimes. Fecundity typically increased with increasing temperature for all species, and hatching success was suppressed at 5°C and 10°C in all species. With respect to salinity, egg production patterns were more irregular, but some species produced fewer eggs at the lowest salinity tested. At a 24 h incubation period, some differences in hatching success were obtained at the different salinities tested. Results indicate that temperature plays an important role in the population dynamics of the congeneric group Acartia within the system and that salinity changes may also influence the fecundity of the species.