Temporal variation in the virgin biomass and abundance of a single stock of the edible jellyfish, Catostylus mosaicus (Scyphozoa, Rhizostomeae) was estimated at Lake Illawarra, a coastal lagoon in New South Wales, Australia. Density and biomass were estimated three times within 3 weeks during three survey periods: April 1997, October 1997 and May 1998. Density and biomass were estimated for two size classes: all medusae easily counted from a moving boat (i.e. medusae =50 mm bell diameter (BD)) and those considered to be vulnerable to the fishery (=200 mm BD). Densities of medusae =50 mm BD varied greatly between survey periods and between days within survey periods. Densities of medusae =200 mm BD were similar between survey periods but varied within survey periods. The virgin biomass of medusae =50 mm BD ranged between 1831 t during April 1997 and 18,526 t during May 1998. The biomass of medusae that was vulnerable to the fishery varied between 207 t in May 1998 and 2865 t in April 1997.