The biology of reproduction in two British pulrnonate slugs. The biology of reproduction has been studied in two British slugs, Deroceras reticulatum (Muller) and Anon hortensis (Frussac). The morphogenesis, histology and ultrastructure of the carrefour and anterior genital ducts have been described and discussed in relation to their function. The carrefour morphology of other slugs has been examined. The limacid carrefour is comparable to the basic stylommatophoran arrangement but in the Anionidae it is much simplified. It is concluded that the Arionidae could be an early offshoot from the main line of stylontmatophoran evolution. Evidence for the environmental control of reproduction in pulmonates has been reviewed. Experimental studies indicate that reproductive development and the onset of maturity are controlled by photoperiod. Short days retard and long days accelerate growth and reproductive development. Subsequent reproductive activity is unaffected by photoperiod. Temperature, humidity and food availability have an immediate effect on egg-laying. In general, conditions favourable to the survival of the embryos enhance egg-production. None of these factors induce courtship and copulation but, generally, adverse conditions are inhibitory. Under apparently constant conditions the animals displayed periods of reproductive activity suggesting an endogenous rhythm. This may contribute to the synchronization of breeding cycles in the field. The behavioural and functional changes during courtship, copulation and egg-laying have been described. During courtship the sperm's movement along the reproductive tract follows a consistent chronological pattern. At copulation the sperm packages are inserted directly into the entrance of the bursa copulatrix, but only sperm released before the package has completely entered the bursa travels up the reproductive tract. This sperm is not stored in the carrefour but passes through into the seminal vesicle where it mixes with the animal's own sperm. At egg-laying the oocytes accumulate in the carrefour although fertilization does not necessarily occur there. The incidence of self-fertilization in the pulmonates is discussed.