Trends in the distribution, population dynamics and reproductive biology of the estuarine cirolanid isopod, Cirolana fluviatilis, were investigated over a 12-month period in the warm–temperate Gamtoos River Estuary, by means of baited traps. Tolerance of adult isopods to combinations of salinity and temperature, and sediment preference, were investigated in the laboratory. Isopods were captured in highest numbers in the lower to middle reaches of the estuary, which appears to be related to a preference by C. fluviatilis for sediment with high mud content. Size-frequency distributions tended to be unimodal, becoming slightly bimodal only after manca release, which occurred predominantly during the warm summer months (December–March). Female C. fluviatilis reached sexual maturity at a length of about 6.0 mm, and males at about 5.2 mm. Fecundity was positively correlated to female body length. Brooding females remained inactive, leading to temporal variations in the male to female ratio.