Female flowering and acorn development in three species of which oaks were studied in Pennsylvania from April 1949 through August 1962 with emphasis on the effect of weather on floral development and acorn production. Pistillate flowers emerge in late April or early May, 5—10 days after the appearance of staminate flowers. After pollination and fertilization, developemnt of acorn forms begins, and filled acorns mature late in August. Spring air temperatures strongly affect acorn yields. Good white oak acorn crops were obtained in years when a warm 10—day period occurred late in April, with average night temperatures 55—60°F, followed by cooler periods (45—50°F) for 13—20 days early in May. During years when April remained cool and was followed by warm days and nights in May, acorn production was poor. Wind, precipitation, relative humidity, and vapor pressure deficits did not significantly influence acorn yields. While some individual trees were inherently copious acorn producers, others in the same stand produced none or a light crop throughout the 14—year period.