The distribution, ecology, and host-plants of the European skipper, Thymelicus lineola (Ochs.), are described, and notes given on its biology and abundance in continental Europe. Agricultural practices are responsible for its virtual absence from hay-fields; however, even in undisturbed wild host-plant communities, it is never abundant, and has never required control measures as in Canada. Parasites of T. lineola are listed, with notes on their life-histories, distribution, host spectra, relative abundance, annual fluctuations, and interpopulation variations. Parasites reared were Phryxe vulgaris Fall., Thecocarcelia incedens Rond, (both attacking larvae, but sometimes emerging after host pupation), Rogas tristis Wesm. (larval parasite), Labrorychus delarvatus Grav. (larval–pupal parasite), Stenichneumon scutellator Grav. and Brachymeria intermedia Nees (pupal parasites); an undetermined mermithid was reared from larvae and also once from an adult S. scutellator. Rogas tristis was attacked by the hyperparasite Mesochorus macrurus Thoms., and S. scutellator by Gelis cursitans F. Potentialities of the more important species for biological control of T. lineola in Canada are discussed. S. scutellator, which is well synchronized with the host, regularly parasitizes 30–50% of pupae and is the most promising natural enemy for introduction, although in Canada it will compete with the native Itoplectis conquisitor (Say). R. tristis (3–57% parasitism) and P. vulgaris (8–44%), with high constancy in Europe, might be considered for introduction to Canada because of the absence there of effective native larval parasites. T. incedens, although probably a specific hesperiid parasite, is uncommon (usually 1–5% parasitism) and has low constancy in central Europe. All other species encountered are rare on T. lineola. Comparison of parasite complexes in Europe and Canada, where the host has acquired 22 natural enemies, shows that four genera are represented in both areas; of three holarctic species two have been found associated with T. lineola only in Canada, and one only in Europe.