This thesis is concerned with intraspecific competition among damselfly larvae, and on the behaviour and ecology of adults. Pyrrhosoma nymphula (Sulzer) was the subject of the work on adult emergence and behaviour at two ponds in Cheshire, while larvae of Ischnura elegans (van der Linden) were used in laboratory competition studies. An asymmetry was found in the way interference competition acts when small and large larvae of I. elegans were contained together with unlimited prey. Small larvae suffered significantly reduced development rates and size increases at the moult in the presence of large larvae, but similar reductions were not apparent when small larvae were combined with like-instar larvae, or when large larvae were combined with small larvae. In field populations, larvae which have been adversely ..rfected by competition for food or fishing sites ".", be smaller and emerge later than competitively successful larvae. Adult size and seasonal occurrence may have a strong influence on both male and female reproductive success. The emergence of P. nymphula was studied at two adjacent ponds. The emergence from both ponds appeared less well synchronised than might have been expected. There is likely to be a trade-off between the need to emerge around the same time as conspecifics in order to achieve a full rep.roRuctive potential, and the expression of temporal variation arising in part frony~petition. Newly emerged adults from one pond were significantly larger and emerged earlier than those from the other pond. This may have been due to differences in prey availability and distribution, and to temperature differences. A decline in size of newly emerged adults through the season was found at both ponds. During periods of bad weather, numbers of individuals emerging were substantially reduced. Those individuals still attempting emergence presumably did so because of the progression of metamorphosis. Quantification of emergence mortality attributed to different factors was attempted. Mortality was found to be highest after a sudden deterioration in the weather. Mean daily mortality, expressed as a percentage of the total daily emergence, was calculated to be 27.93%. There was no evidence of density-dependent mortality at emergence. Recovery of individuals marked at emergence and when mature was found to be low. For mature adults, this was attributed predominantly to dispersal to nearby water bodies. There was some indication that there was a higher mortality of smaller adults in the immature and reproductive stages of the life-history. Fewer females revisited the ponds than males which led to the estimation of female survivorship being considerably lower than that of males. A large increase in weight of females from emergence to maturity suggests that they may amass clutches of eggs over several days and thus not visit the ponds on every day of their reproductive life. In addition, mortality of females may be higher than that of males as a result of predation during oviposition. For male P. nymphula, size was shown to have no effect on the ~:)Utcomeof territorial disputes.which were settled by a simple residency asymmetry. SIze was also found to be unrelated to the total number of observed matings a male achieved, and the number of days a male spent at the ponds, although the latter may have been obscured by the low return of marked males. The relation of size to longevity could not be studied. Success in mating and in territorial disputes was found to be positively related for resident males. This was probably due to the amount of time spent at the ponds within a day. Late emerging female P. nymphula visiting the ponds later in the season laid smaller clutches of smaller eggs than those arriving early in the season, irrespective of size. Clutch size is likely to be strongly related to the inter-clutch interval and thus the pattern of sunny and cloudy days through the season.