The size of prey preferred by a range of sizes of Carcinus maenas was tested by feeding crabs daily with 5 individuals of each of 3 size groups of either Mytilus or Mercenaria. The number of prey of a given size eaten per day was not unduly affected by altering the size structure of the population. Therefore estimates of the size of prey eaten at a rate of 1 per day give useful indications of the size at risk. The number of prey eaten was affected by a number of factors. Enlarging the number at risk increased the predation rate and selection was biased towards the smaller prey offered. Mercenaria in plain bins were eaten in larger numbers than those which were allowed to bury in sand, while those buried in mud were eaten the least. Crabs collected from Llandudno were more voracious than those from Conway at all seasons of the year. A number of trials were made at ambient temperature at all seasons of the year. Some mussels were eaten at water temperatures as low as 2·3°C; the maximum numbers were eaten within the period May-September. During the experiments 162 crabs moulted, and the records of the increment in carapace width showed that it became larger as the premoult size increased.