On each of two former occasions when specimens of Gammarus chevreuxi Sexton have been brought from the wild into an incubator and kept at 21° C. or more, red-eyed recessive types have been reared among the F2 progeny (4, pp. 190 and 194). In November, 1930, an opportunity occurred for making further tests on this point. Twenty-three wild pairs, taken in Chelson Meadows on November 19, were placed on the following day in the incubator, the temperature of which at first averaged between 21° and 22° C, but was raised after a few days to an average of between 22° and 24° C. The young which were extruded from eggs laid in the wild were discarded. Those of subsequent broods were reared with a view to obtaining as many F2 families in each stock as would give a reasonable opportunity for segregating recessive characters to appear. The F2 young were examined for recognisable variations that might indicate the presence of a recessive character.