Field observations on the site of oviposition by Eulachnus agilis (Kaltenbach) showed that the majority of eggs were deposited singly in leaf scars on branches of Scotch pine, Pinus sylvestris L. The development of the fundatrices and virginoparae under controlled laboratory conditions was investigated. The times required for the fundatrices to complete a full generation were 11.5, 19.2, and 26.7 clays at the respective temperatures of 21.1°, 15.6°, and 10.0°C and a photoperiod of 12 hours. A generation of the virginoparae was completed in 13.3, 21.6, and 45.3 days at the same respective temperatures as the fundatrices and a photoperiod of 14 hours. The mean number of nymphs produced by the fundatrices was 23.5, 17.8, and 16.7 at the respective temperatures of 21.1°, 15.6°, and 10.0°C. At the same temperatures, the virginoparae produced 8.3, 13.5, and 6.2 nymphs. At each temperature, the fundatrices produced a significantly greater number of progeny than the virginoparae.