Development and mortality of the potato leafhopper [Empoasca fabae(Harris)] were studied in the laboratory under constant temperature regimes. Lower thresholds for egg and nymphal development were 7.6 and 8.8°C, respectively, and the upper threshold for development was ca. 29°C. Nymphal mortality was high at 13°C, decreased at 18°C, and gradually increased to 81% at 35°C. Most mortality occurred in the first and fifth instars. Adult measurements indicated differences in size of head capsule width and body length due to sex, differences in whole length due to sex and temperature, and differences in wing length due to temperature. Centigrade degree day (CDD) accumulation for egg development was related to field monitoring of alfalfa for leafhopper eggs, nymphs, and adults during the spring of 1976 and 1977 in Montgomery Co., VA. Calculated CDD from first females collected to nymphal hatch and from first observed eggs to nymphal hatch was 199 ± 20.4 and 136±39.7 respectively. This system could be used in an alfalfa scouting program to indicate when nymphal samples should be collected based on accumulated temperatures after arrival of female potato leafhoppers in the spring.