The first oxygen isotope analysis of biogenic opal from lake sediments, from the Aller¿d/Younger Dryas transition in a core from Linsley Pond, Connecticut, gives an average estimate of a 6 ¿C drop in temperature during the Younger Dryas. This shift represents temperatures during the bloom season, and may be less than the winter temperature drop. The sharp transition itself, with a duration of about 200 years, suggests that the temperature decrease may have been as large as 12 ¿C. Previous estimates of the Aller¿d/Younger Dryas temperature shifts are controversial, and range from 3--20 ¿C, suggesting that further interdisciplinary research on the same samples is warranted. ¿ 1998 American Geophysical Union