Internally fertilizing hermaphroditic animals show a bewildering variety of mating behaviours and mechanisms and many species have reciprocal sperm exchange. When matings are frequent and costly, partners are predicted to donate more sperm when they receive more sperm (i.e. conditional sperm exchange), but this has been shown experimentally only in sea slugs, free-living flatworms and possibly cestodes. We studied mate choice and sperm reciprocity patterns in the hermaphroditic land snail Succinea putris. Mate choice was random with respect to shell size. However, in matings where the partners were of unequal size, significantly more matings were observed where a small active individual mated on top of a larger inactive individual than the reverse. We suggest that this may be the result of mating on a vertical substrate or hanging underneath a horizontal substrate, because under these conditions it may be easier for a larger, inactive individual to carry a smaller, active one, than vice versa. The number of sperm transferred during mating was highly variable and was not related to the size of the donor, the size of the recipient, the size difference between the two partners or mating duration. Twelve of the 87 matings involved unilateral sperm exchange. There was no evidence for conditional sperm digestion. There was a significant, but weak positive relation between the numbers of sperm donated by the two partners. Hence, it remains unclear whether sperm exchange in S. putris is conditional or not. © 2005 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.