The population structure of the commercially harvested jellyfish Catostylus mosaicus (Scyphozoa, Rhizostomeae) was investigated in estuaries and bays in New South Wales, Australia. Variations in abundance and recruitment were studied in 6 estuaries separated by distances ranging from 75 to 800 km. Patterns of abundance differed greatly among estuaries and the rank abundance among estuaries changed on 5 out of the 6 times sampling occurred. Great variation in the timing of recruitment was also observed among estuaries. Variations in abundance and recruitment were as extreme among nearby estuaries as distant ones. Broad scale sampling and detailed time series of abundance over a period of 2.7 yr at 2 locations showed no consistent seasonal trend in abundance at 1 location, but there was some indication of seasonality at the second location. At Botany Bay, the abundance of medusae increased with distance into the estuary and on 19 out of the 30 times sampling occurred medusae were found at sites adjacent to where rivers enter the bay. Medusae were found to be strong swimmers and this may aid medusae in maintaining themselves in the upper-reaches of estuaries, where advection from an estuary is least likely. Variability in patterns of abundance and recruitment suggested regulation by processes occurring at the scale of individual estuaries and, combined with their relatively strong swimming ability, supported a model of population retention within estuaries. Populations of C. mosaicus in individual estuaries and saline lakes in New South Wales should, therefore, be considered as separate stock units.