We investigated the role of filament length of the toxic cyanobacterium Cylindrospermopsis raciborskii on the grazing of Daphnia magna by providing different food mixtures. It was hypothesized that longer C. raciborskii filaments would reduce the clearance rates of D. magna more than shorter ones. Also, C. raciborskii was expected to have a stronger negative impact on the grazing of larger animals. The clearance rates of two D. magna size classes (smaller, 2.0 ± 0.1 mm and larger, 3.1 ± 0.06 mm) were measured in short-term laboratory grazing experiments using individuals fed with two different monocultures of a saxitoxin producer C. raciborskii strain distinguished by filament lengths (average 137 and 61 µm). The cyanobacteria were offered as the sole food source and as a mixture (1:1) with Scenedesmus obliquus. A treatment with only S. obliquus was also applied. The length of C. raciborskii filaments did not have a clear influence on the clearance rates of D. magna within the range we tested. Thus, longer C. raciborskii filaments may not necessarily cause stronger feeding inhibition than shorter ones. Most of the grazing reduction occurred when C. raciborskii was the sole food for either size of D. magna. Larger animals experienced increased feeding inhibition by the addition of C. raciborskii to the diet. The proportion of C. raciborskii relative to suitable food may be a key factor for the outcome of feeding performance of D. magna. We encourage other authors to test the validity of these conclusions over a larger range of filament lengths and on strains with different biochemical makeup.