Cannibalism is a very common phenomenon throughout the animal kingdom. Females of the green dock leaf beetle, Gastrophysa viridula, lay clutches of eggs on the leaves of dock plants (Rumex species). Cannibalism takes the form of larvae eating unhatched conspecific eggs. Both within-clutch and inter-clutch cannibalism were observed. Within-clutch cannibalism was found to be of very limited extent. However, two observations indicated that cannibalism in G. viridula is more important than this finding might suggest. First instar larvae proved to be much more voracious cannibals when given an unlimited supply of newly laid eggs than when in normally hatching clutches. Also, viable as well as non-viable eggs were eaten from within clutches. The individual fitness benefits gained through cannibalism were studied. Larvae fed on eggs alone were unable to complete the first instar. But larvae given young eggs during their first instar along with dock leaves throughout all three instars did better in terms of survival and developmental rates than larvae fed only on dock leaves. No relationship was found between adult female size and lifetime egg production or longevity. Females were found to lay consecutively larger and smaller clutches throughout most of their lives. Mathematical models were used to interpret the results of a sperm competition experiment. The results were consistent with two mechanisms of sperm competition: linear transfer of sperm with time from male to female and sperm removal or repositioning with sperm mixing, or diminishing transfer of sperm with time and sperm mixing without displacement. The individual benefits of cannibalism in terms of survival observed in the laboratory were sufficient to satisfy Hamilton's inclusive fitness criterion for the evolution of selfish behaviour, even for the case of larvae eating full-siblings. It was anticipated that mechanisms may have evolved to reduce the risks of inter-clutch cannibalism. The distance from previously laid clutches that females lay their eggs was found to be important in this respect. and an intrinsic protective mechanism against cannibalism which becomes more effective with age was found. A possible link between this mechanism and chemical defence normally assumed to have evolved as a mechanism against inter-specific predation is suggested. It was found that the relative concentration of total oleic acid increased with age in G. viridula eggs, whereas the relative concentrations of all other major fatty acids declined or remained constant. The possible anti-cannibalistic role of oleic acid merits further investigation. The relationships between clutch size, asynchrony of hatching and the extent of within-clutch cannibalism were also studied. Asynchrony of hatching was found to increase with clutch size. Although negative relationships were found between hatching success of viable eggs and both clutch size and asynchrony of hatching, multiple regression showed that clutch size was the primary factor affecting the hatching success rate of viable eggs. An ability to distinguish between related and non-related eggs as victims was not found among first instar larvae. This finding is discussed in the light of individual and kin selection and the probability of encountering related eggs.