Assessing the colonizing ability of a species is important for predicting its future distribution or for planning the introduction or reintroduction of that species for conservation purposes. The best way to assess colonizing ability is by making experimental introductions of the species and monitoring the outcome. In this study, different-sized propagules of Roesel's bush-cricket, Metrioptera roeseli, were experimentally introduced into 70 habitat islands, previously uninhabited by the species, in farmland fields in south- eastern Sweden. The areas of introduction were carefully monitored for 2-3 yr to determine whether the propagules had successfully colonized the patches. The study showed that large propagules resulted in larger local populations during the years following introduction. Probability of colonization for each propagule size was measured and showed that propagule size had a significant effect on colonization success, i.e., large propagules were more successful in colonizing new patches. If future introductions were to be made with this or a similar species, a propagule size of at least 32 individuals would be required to establish a viable population with a high degree of certainty.