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The type of food given during short chronic assays with the rotifer Brachionus calyciflorus might be one of the sources of variation in the reproductive rate of the rotifers. Ten green algal species were supplied as monospecific diets to examine variability in rotifer growth rate. In addition, rotifers were fed cyanobacteria as monospecific food and in mixtures with the green alga Scenedesmus obliquus to test the hypothesis that cyanobacteria are a valuable supplement in combination with food items such as Scenedesmus. The population growth rate of B. calyciflorus was influenced significantly by the different green algal food species. S. obliquus promoted the highest growth in B. calyciflorus (1.6per day), while growth on Desmodesmus abundans was the lowest (0.3 per day). As percentage of maximal growth on S. obliquus, the sequence of green algal food species is: Desmodesmus subspicatus (88%), Chlorella vulgaris (83%), Monoraphidium minutum (77%), D. quadricauda (74%), S. falcatus (71%), S. acuminatus (69%), S. pectinatus (64%), Chlamydomonas reinhardtii (57%) and D. abundans (19%). The growth differences were not explained from size differences of the algae. The cyanobacteria Microcystis aeruginosa and Synechococcus elongatus depressed growth of B. calyciflorus not only as monospecific food but also in mixed diets with the good food source S. obliquus. The detrimental effect appeared not related to microcystins. The results do not support the hypothesis that nontoxic cyanobacteria are a valuable supplement in combination with other green algal food species. The variation in growth rates observed in current study underlines the influence food species might have on the growth of B. calyciflorus and potentially on the outcome of toxicity tests