The systellommatophoran slug Veronicella sloanii (Cuvier, 1817), is a simultaneous hermaphrodite. This slug is an agricultural and horticultural pest in Barbados and several islands of the Lesser Antilles. Over the period January-July 2006 and June-August 2010, the mating behavior of this species was determined by ad libitum and focal animal sampling of captive slugs collected from six sites on the island of Barbados, supplemented by observations and length measurements of slugs seen mating in the field. Individuals of Veronicella sloanii mated reciprocally in pairs, but also in a multi-partner ring formation involving three individuals. Two stages in the mating process were identified, courtship and copulation. Courtship was short, less than two minutes in mating events that led directly to copulation (mean 1.87 minutes, range 0.25–2, N = 53). Copulation in contrast was long, lasting on average 1.03 hours (range 0.4–2, N = 40). During mating the penial gland of each partner made contact with the foot or the hyponotum of the other partner. Aggressive behavior during mating in this slug was manifested by non-mating individuals pushing themselves between mating pairs resulting in the withdrawal of the penis of the mating pairs and cessation of copulation. A strong size-assortative mating pattern was observed; individuals in mating pairs were of similar size.