A method is described for the laboratory breeding of the rabbit flea in which the immature stages are reared at constant temperature and humidity. Eggs are obtained by confining fleas taken from a rabbit and her nest shortly after parturition with two of her nestlings in an incubator for 24 h. The eggs are transferred to an artificial diet medium on which the immature stages are reared. On average a female flea produces 50 eggs during the first six days post-partum. At 25 degrees C, 95% of eggs hatched at 79% RH and 98% at 84% RH. Most eggs hatched on the third day after laying and hatching was completed by the fourth day. Significantly more fleas of both sexes were obtained when larvae were reared at 25 degrees C on a medium containing powdered 41B rodent diet than on one containing terrier meal. Both diets also contained yeast and dried rabbit blood. There was no significant difference between the numbers of fleas obtained at 79% RH and 84% RH. Significantly more fleas were also obtained when larvae were reared at 27 degrees C, 84% RH, than at 25 degrees C. Female fleas emerged sooner than males at both 27 degrees C and 25 degrees C. Fleas from the laboratory culture were heavier than those from wild nests. Female fleas were heavier than male fleas in both cases.