Mating in Deroceras consists of an investigation phase (precourtship), then a long courtship involving mutual stroking with the extruded sarcobelum, then sperm exchange (copulation). The penial gland, if present, everts over the partner's skin during copulation: this is hypothesised to apply a secretion manipulating the partner to use received sperm. Deroceras gorgonium has a particularly large penial gland divided into many finger-like branches. We studied D. gorgonium mating behaviour in the hope of further indications of the gland's function. Precourtship and courtship together last longer than in other Deroceras (ca. 6 h to >9 h); precourtship is highly variable, often with many bouts of different behaviours, including seemingly inactive phases. During most of the courtship partners remain apart waving their particularly long, pointed sarcobela; only at a later stage do the tips of these contact the partner. This waving alternates with circling for half a turn. For the first time in Deroceras we observed the sarcobelum transferring a secretion. The copulation is amongst the fastest: genital eversion and sperm exchange occur within 1 s, and slugs separate 18-25 s later. The penial gland is everted immediately after sperm exchange, but, surprisingly, is often spread underneath the partner rather than over its back and, if on top, is not always fully spread over the partner's body. We discuss these observations with respect to penial gland morphology and in the light of possible sexual conflicts. The long courtship and distant sarcobelum waving might reflect attempts to transfer, but not receive, secretion, and the circling might serve for size assessment.