The biology, and in particular the embryonic development, of two species of Chesias (Lepidoptera: Geometridae) are described and compared. Some aspects of the general biology of these species are examined, and these include the time of occurrence of the different stages of the life cycle, the behaviour (particularly during oviposition) of the adults, and the parasites attacking the larvae. The morphology of the developing embryo is described in detail, and comparisons between the two species are made. Morphogenesis is divided into a number of arbitrary stages, and the relative duration of the different stages is compared. The temperature relations of the developing embryo are examined in detail in both species. In particular, the changing temperature requirements of the embryo of C. leqatella, which diapauses at an early stag-,are determined by the examination of large samples of eggs killed at different times during embryonic development. The existence of parental effects on the embryonic development of the progeny is also investigated, and certain aspects are discussed.