Five species of Arion slugs were collected repeatedly at a woodland site in southern England and all individuals weighed. Selected samples of these were dissected so as to weigh components of the reproductive tract. The relative weights of the gonad, spermoviduct and albumen gland provided the basis to categorize individuals into adult, subadult or immature classes, or as juvenile if the sum of these weights was below a threshold. This procedure was validated by raising A. subfuscus in captivity and killing at a range of known ages before and after egg laying. In the other species, organ weights from individuals observed to have laid eggs or mated also helped to calibrate the divisions. Such data from two species demonstrated that, following the production of an egg clutch, the albumen gland took days gradually to regrow. There was little evidence of much variation in life cycle from year to year and the broad patterns, although not precise timings, agreed with studies elsewhere. No species produced more than one generation per year and in all there was a season (brief in A. subfuscus) when adults were absent. The life cycles were predominantly annual, although in some species a minority of individuals might take 18 months to mature. The time of year at which individuals matured into adults varied between species: A. intermedius in August and September, A. distinctus mostly in December and January, A. circumscriptus mostly January to April, A. subfuscus April to early October and A. rufus July to September. The largest two species thus dominated in summer, but at other times the species overlapped considerably in size. In four species, individuals maturing later in the season did so at a smaller size; the possible exception was A. intermedius, in which maturation was highly synchronized. The coefficients of variation in adult size were compared against a collection of such data from other terrestrial molluscs. The smallest species, A. intermedius, had disproportionately large hatchlings.