In promiscuous species, multiple sites of sperm storage are considered to be an adaptation of females for post-copulatory control over sperm storage and selection to control offspring paternity. 1,2 In the stylommatophoran gastropod Arianta arbustorum (Linnaeus), the sperm-storing spermatheca consists of two to nine blind tubules: a main tubule and one to eight lateral tubules. 3,4 The considerable variability in paternity patterns of double-mated individuals and the observation that incoming sperm are not equally distributed among spermathecal tubules, i.e. some are filled and others remain empty, suggest that A. arbustorum may store and use sperm from different mating partners selectively. 3,5 In addition, ultrastructural analyses of the spermathecal epithelium with its ciliation and of the musculature surrounding the spermatheca indicate that anatomical correlates necessary for sperm manipulation exist. 6,7 The sperm storage sites themselves may be subject to sexual selection, provided that there is heritable variability of those structures, which are important for exercising cryptic female choice. 8,9 In a previous study, we investigated whether different risks of sperm competition, which we expected in populations with different densities, would affect spermathecal morphology. There may be several reasons why we did not detect any density dependence including lack of heritability. 4,10 Instead, the number of spermathecal tubules might depend on an individual’s age or state of sexual maturity. The ontogeny of a complex spermatheca such as that of A. arbustorum has not been investigated in much detail so far. It has only been observed 11 that in A. arbustorum the fertilization pouch (sensu 3 ), of which the spermatheca is one part, develops at an age of about 6 weeks. However, the development of the structures of the fertilization pouch including the spermatheca was not studied so far. In the present paper we investigate the development of the spermatheca in A. arbustorum. We were particularly interested in whether the number of spermathecal tubules increases with age/size (see below) or whether the anlage of all spermathecal tubules is formed during an early stage of ontogenesis. The latter would be an indication that the complexity of the spermatheca is mainly genetically controlled and, thus, heritable. A. arbustorum is common throughout Europe from the Pyrenees to the Ukraine reaching northwards as far as Scandinavia. The species occurs from the lowland up to 3000 m a.s.l. 12,13 The snail has determinate growth indicated by a reflected and thickened lip. Formation of the lip and sexual maturity roughly coincide at an age of 2‐4 years, and adults live another 3‐4 years. 14 A. arbustorum mates repeatedly during the