The effect of temperature (T) on filtration rate (F) in filter-feeding bivalves is controversial. Some studies show a clear increase in F with increase in T, in agreement with decrease of kinematic viscosity with increasing temperature and simultaneous increase of beat frequency of the water-pumping lateral cilia. But other studies show little or no effect. Here, we present our understanding of the effect of T on F in the blue mussel, Mytilus edulis, primarily based on own previously published experimental studies. We point out the possible pitfalls for little or no effect of T on F, most likely caused by erroneous use of the method for measuring F and failure of separating the factors that may affect F, e.g., the temperature-tolerance interval, and the algal-concentration interval within which the mussel is optimally filtering at a specified T.