SUMMARY.The speciesP. hyoscyamihas been recorded at various times by various authors, and it has often been described under different names, partly because of its having been reared from a fairly wide range of food plants. The belladonna leaf‐miner is the larva of this species, found during the summer throughout Europe, United States of America and Canada.The injury to the plant consists in the destruction of the parenchyma which the maggot greedily devours, the leaves assuming a blistered appearance in consequence. The leaves thus attacked quickly flag and wither during dry weather. In this way excessive damage to the various food plants often results in their total loss, heavily affecting the agriculturist pecuniarily where they are grown as cultivated crops.Other food plants besides belladouna are mangolds, beet and henbane.The number of the larvae in one leaf varies with the size of the latter and, roughly speaking, directly as the size.The ravages are periodic and often quite localised, resulting in diminished yields of the products of the different crops attacked. The top shoots are most heavily infested early in the season, but later the radical leaves are most attacked.Hibernation occurs in the pupal condition about two inches below the surface of the soil near the food plants.The number of broods vary. There are at least three in latitude. The broods are not separated sharply off from each other. There is a good deal of overlapping so that all stages occur in the field during the greater part of the season.The eggs are deposited superficially on the back of the leaf in groups consisting of parallel series varying in number. The incubation period is about 5 days.The larvae feed uninterruptedly and complete their metamorphosis in 10 days under the most favourable circumstances. The larvae of the first two broods sometimes pupate in the leaf, generally making. their way to the margin to do so. The pupal period of the first two broods is about 17 days.The average period for one complete life‐cycle is about 36 days.Two closely related species,P. bicolorandP. nigrit arsis, attack common weeds such as dock. Their life‐histories are, in all details, almost similar to that ofP. hyoscyami.Structurally, there are some interesting differences, especially in the larval stages.According to the different food plants which it affects,hyoscyamimay be divided up into at least two “biologic” species, one of which would seem to confine its energies to a few members of the Cheno‐podiaceae, the other to Solanaceae, and within these two families preferences to different species are shown. But in the absence of the one favoured food plant, another, not ordinarily so attractive, may be selected.Species of the Chenopodiaceae and Solanaceae have in common certain specific organic substances belonging to the group known as the alkaloid bases. They probably serve as an attraction to the fertilised females to oviposit on the leaves of the plants which contain the active principles concerned.Experiments showed that mangold‐reared adults would not oviposit on belladonna andvice versa.This restriction to one kind of plant is indirectly advantageous to the agriculturist in that strains of flies reared on belladonna confine themselves probably to this species or one closely related, such as henbane, and do not attack mangolds.The young plants are more easily killed than the more advanced ones.Natural control of the pest is secured by the parasitism of two species of Braconids on one or both of which a Proctotrypid is probably hyper‐parasitic.The degree of parasitism ascends to a climax at the end of August and beginning of September, and then suddenly diminishes.Frequent hand‐picking of attacked leaves and their destruction provides a ready and effective means of killing the maggot and unhatched eggs. This method is only practicable where the crop is a small one.Dressings of stimulating, chemical manures in the early Btages, strengthens the plants so that they maintain themselves the better against the injurious effects of infestation.Farmyard manure which attracts the flies should where used be applied in the autumn to give it the chance of decaying before the adults appear in the spring.Deep‐ploughing in the autumn serves to bury the hibernating puparia which lie near the surface, thus rendering the emergence of the adults in the spring a matter of comparative difficulty.Paraffin emulsion is not so effective in killing the maggot as this same emulsion