Phylomycus carolinianus (Bosc, 1902), (Mollusca: Gastropoda: Philomycidae) also known as Carolina mantleslug, is a widespread but poorly known terrestrial mollusc. We conducted studies to assess key aspects of its natural history. In the laboratory, its pattern of growth followed a sigmoid curve, but the growth rate was highly variable. Using a hierarchical clustering analysis of time to achieve reproductive maturity, the individual slugs could be separated into 4 discrete developmental groups: fast, intermediate, and slow-growing individuals, and also some that failed to develop completely. The 3 groups of developing slugs achieved reproductive maturity in about 129, 173, and 217 days, respectively. This developmental polymorphism suggests intra-generation risk-spreading. Reared alone or in pairs, slugs produced eggs in about 6 months; eggs numbered about 65 per cluster, though paired slugs produced slightly more eggs per cluster. Thus, these hermaphroditic slugs are capable of self-fertilization. Eggs hatched in about 3 weeks. Embryonic development occurred across the entire temperature range (10–29 °C) tested. The proportion of embryos developing was higher at 10, 14, 17 and 21 °C than at 25 and 29 °C. However, the embryos that developed at 10 and 29 °C did not hatch. Eggs incubated at 14 °C had the longest pre-hatching period, with those held at 25 °C the shortest. Synthetic gypsy moth and spruce budworm diets, and white mushrooms, all favored weight retention by adult slugs more than some natural diets tested, though mature slugs fed several diets produced eggs. Culture conditions of 21 °C and either gypsy moth or spruce budworm diet seemed optimal for growth and survival. Nearly all of the 51 mushroom species (representing 18 families) presented to slugs were eaten, though some much more readily than others. Except for Romaine lettuce, none of the foliage from 37 green plants offered were accepted as food.