This study demonstrates the feasibility of radioisotope tagging of the sheep tick, Ixodes ricinits (L.). Details of oviposition, hatching, and radioactivity in eggs and larvae of this species are described. The sheep tick, Ixodes ricinus (L.), is a major vector of tickborne diseases throughout its range. Although the ecology of this tick has been studied extensively, (Campbell, 1950; Lees, 1948; MacLeod, 1939; Milne, 1943, 1945 a, b, 1947; Walton and ODonnelll, 1969), knowledge of several ecological characteristics which may influence its ability to spread disease is still lacking. Tick population density, the degree of dispersal by domestic animais, overwinter survival, and survival of population cohorts are among the important ecological attributes of the tick which must be analyzed quantitatively in order to understand the persistence and spread of disease by I. ricinus. Radioecological methods (i. e., ecological tracer technique) appear to be especially suited to such fields studies. Radioisotope tagging of ticks was proved feasible for 5 species (Sonenshine and Yunloer, 1968), and the technique was used in a study of the ecology of Dermacentor variabilis (Say) (Sonenshine, 1972). Nevertheless, one cannot conclude that this technique is equally useful for al! other ticks. This paper reports the results of laboratory studies to determine whether adequate radioisotope tagging of I. ricinus larvae was feasible.