Whilst investigating the biology of parasites of the frit-fly (Oscinella frit, L., Diptera, Chloropidae) in North America with a view to introducing possible species for the biological control of this pest in Great Britain, extensive experimental work was done with one species of Chalcid, Spalangia drosophilae, Ashm. (Spalangidae), a parasite of frit puparia. During this work conditions inducing a state of diapause in the mature larvae were investigated. As with many other species, it was found that low temperatures during development favoured entry into diapause, whilst high temperatures favoured emergence of the progeny without arrest in development. However, other factors were found to have some important bearing in this respect. Some individual females produced progeny, all of which entered a state of diapause in the full-grown larval stage, others bred and ovipositing under identical conditions produced progeny all of which emerged without arrested development, others produced progeny that were mixed in this respect. This effect did not seem to be inheritable. The history of each individual female with respect to environmental conditions was found to have an important influence on the propensities of her progeny for entering diapause. These effects will be dealt with more fully at a later date. There was, however, one striking effect which has apparently not been noted before, and which it was felt was of sufficient interest to be recorded separately.