Female colour polymorphism is a perplexing characteristic of many damselfly species. In Ischnura elegans three female phenotypes occur, one of which has the same blue coloration as the male (androchromes) whilst the others are inconspicuous brown gynochromes (infuscans and infuscans-obsoleta morphs). By marking a natural population near Rome, Italy, we found that all female phenotypes have similar survivorship, but they differ in mating frequency. Androchromes represented 55% of females but were involved in 43% of matings, whereas infuscans females represented 27% of females and 40% of matings and the infuscans-obsoleta phenotype 18% of females and 17% of matings. Old androchromes stored significantly less sperm in their spermatheca than old gynochromes, suggesting that they had mated less often. The majority of mature androchromes were observed alone (54%) when the majority of gynochromes (82-84%) were mating. When live tethered conspecifics were presented to males, blue models (male and androchrome female) were less attractive than brown models (gynochrome females). In contrast, all female colour morphs and males were equally (highly) attractive to males when the models were dead. Androchromes were significantly larger than gynochromes. Our results indicate that androchrome females mate less often than gynochromes, which could be a means of avoiding unnecessary and costly matings, but some androchrome females failed to reproduce (mate or oviposit) probably because they were unable to mate at all. The different explanations for the maintenance of this polymorphism in I. elegans are discussed. Copyright 1998 The Association for the Study of Animal Behaviour.