Small ephemeral pools in the Murray-Darling basin, Australia, that dry annually lack fish and are dominated by microfauna. Community size structures and species diversity are very variable and many ponds contain one or more of the large calanoid copepods, Boeckella major, Boeckella pseudochelae and Hemiboeckella searli. Gut content analysis showed that these species were omnivores with B, major being the most carnivorous and B. pseudochelae the least. B. major guts contained 21 animal taxa (cladocerans, calanoid and cyclopoid copepodids, nauplii and rotifers), H. searli contained 15 taxa (calanoid, cyclopoid and harpacticoid copepods, nauplii and rotifers) and B. pseudochelae 8 taxa (small cladocerans, nauplii and rotifers). Mean numbers of animals per gut differed significantly between the large calanoids and in B. major, B. pseudochelae and H. searli were, respectively, 2.9, 0.6 and 0.2. In B. major "Rotifers" were positively selected, "Copepodites" and "Others" neutrally selected and "Cladocera" and "Nauplii" negatively preferred. In H. searli "Copepodites", "Nauplii" and "Rotifers" were neutrally selected, and "Others" negatively preferred. B. pseudochelae showed negative preference for "Cladocera", "Nauplii" and "Others", and "Rotifers" were neutrally selected. At sites containing B. major, species diversity was significantly reduced and community size structure (measured as mean and median log-length, % of sizes >500 mu m) was significantly larger than it was at sites lacking large calanoids or with only B. pseudochelae and H. searli. It is hypothesised that predation by B. major may be an important force structuring the communities of these habitats.