The population biology and habitat of Haliclystus auricula was studied from samples taken every two months in southern Chile between November 2001 and November 2002. A total of 3790 medusae were collected and examined. The abundance of H. auricula varied seasonally. Maximum densities occurred in summer (1405/m2) and minimum densities during the winter, when the population gradually disappeared. Gonads were observed in medusae starting from a size of 2·0 mm. Stauromedusae were found most often on the red filamentous alga Ceramium rubrum (69%), followed by Gymnogongrus furcellatus (12%) and Ulva sp. (10%). There were highly significant differences in mean abundance of medusae on these three algae (P<0·05). The remaining 9% of medusae were found on other seaweeds. The smallest medusae (0·08–0·9 mm umbrella height) were found exclusively on C. rubrum. The highest densities were recorded in the midlittoral zone.