Some aspects of the biology of Pacifastacus leniusculus and Astacus leptodactylus have been compared. The literature survey shows that considerably more studies have been carried out on P. leniusculus than A. leptodactylus. Although no major differences have been found in the morphology of appendages and mouthparts of the species, structural differences have been found in the abundance of setae on the second maxilliped, in the number of teeth on the mandibles and the crista dentata, and form of the chelipeds. Studies on the environmental tolerance of the species show that both species are able to survive in saline water for long periods of time but they can only increase in number in low salinities. Both species can survive over a wide range of temperatures, but they cannot tolerate temperatures of 34 °C after stepwise acclimation. Although the results do not show a clear difference in the tolerance of P. leniusculus and A. leptodactylus to low oxygen, there are some indications that A. leptodactylus is more tolerant of decreased oxygen tensions than P. leniusculus. By using a non-invasive heart beat monitor on crayfish it has been observed that the frequency of heart beats is extremely variable and can be affected by many factors, such as temperature and salinity. Juveniles of the two species can have a significant impact on plant and macroinvertebrate communities. The results also show that both species can have a negative effect on the recruitment of fish populations in freshwaters by eating fish eggs. Competition experiments show that both juveniles and adults of P. leniusculus are significantly more aggressive than those of A. leptodactylus. The results also show that A. leptodactylus would be eliminated by P. leniusculus if they met in a wild. Adults of the species prey on their juveniles, except the brooding females with stage 2s. This predation occurs in the presence of adequate nutrition. Non-predatory behaviour of the brooding females may indicate the presence of pheromones in the species. Reproductive efficiency of the populations of the species in Britain is as good as any studied elsewhere. In comparison to A. leptodactylus, P. leniusculus has more eggs, but smaller in size. Pleopodal egg development of the species can be reduced from seven months to three months with temperature acclimation, but photoperiod is not a factor in reducing pleopodal egg development. In both species sexual dimorphism was observed between males and females. Males of both species and females of P. leniusculus exhibit allometric or isometric growth during their lives but female A. leptodactylus exhibits isometric growth throughout. Comparison of body parameters shows that P. leniusculus can be considered as a morphologically better species to adapt to environmental conditions than A. leptodactylus because it has large and heavy chelae, and heavy body weight. Both species grow fast, but because P. leniusculus hatches earlier it has an advantage over A. leptodactylus and has bigger juveniles by the end of the first summer. In both species males produce significantly more claw meat than females. Although A. leptodactylus produces significantly more tail meat, males of P. leniusculus produce significantly more claw and total meat. Significant differences occur in the tail meat yield of female A. leptodactylus and in the claw meat yield of female P. leniusculus, but males produce similar amount of meat in winter and summer. The Swedish trappy is very effective method of catching both species over a certain size. Day and night catches show that both species are very active during the day and night.