Oedothorax gibbosus is a rare dwarf spider species in Flanders bound to oligo- and mesotrophic alder carrs. This dwarf spider is characterised by the appearance of a male dimorphism; there is a gibbosus male morph with a hunch on the last third of the carapace, anterior to which is a hairy groove, and a tuberosus morph without these features. Earlier studies have already indicated some differences between both male morphs. According to VANACKER et al. (2001) the juvenile phase (the period between the emergence of the spiders and the last moult) of the gibbosus morph is significantly longer than the juvenile phase of the tuberosus morph, and this is presumably necessary for the production of the hunch and the hairy groove. This hairy groove probably secretes a fluid that is important for the gustatoric courtship behaviour and gibbosus would therefore have a reproductive advantage (HEINEMANN & UHL, 2000). In this paper we report another interesting difference between both morphs. To examine if the male morphs also differ in survival, some survival experiments, in normal food and moisture conditions and in two extreme conditions, namely in foodless and low moisture environments, were set up. The gibbosus male is significantly more susceptible to foodless conditions than the tuberosus male, and also in normal conditions tuberosus lives significantly longer than gibbosus. This is demonstrated by means of survival curves. In low moisture conditions, however, we could not prove that survival rate differs between both male morphs. The higher survival rate of tuberoses is a possible explanation for the morph ratio in favour of tuberosus in the studied field population.