The developmental biology of citrus mealybug, Planococcus citri (Risso) (Hemiptera: Pseudococcidae), was studied at constant and varying temperatures on roses, Rosa hybrida L. Mealybug populations were sampled daily to determine thermal constants and day-degrees (DD) at 18.3°C, 26.6 d/15.5°C night and a fluctuating temperature averaging 20.3°C. The thermal constant at 18.3°C was 289 DD for females and 346 DD for males; the median number of days from egg to adult was 39 for females and 47 (±1 SEM) mean days for males. At fluctuating temperatures averaging 20.3°C, the thermal constant for females was 365 DD and 421 DD for males; the median number of days from egg to adult for females was 39, and 45 (±2.0 SEM) mean days for males. For females reared at 26.6°C during the day and 15.5°C during the night, the thermal constant was 326 DD, and the median days to egg hatch, second instar, third instar, and adulthood were 6, 17, 26, and 32, respectively. The lower developmental threshold was determined to be 10.9°C for the egg stage. Information about development of the citrus mealybug on roses is discussed in terms of effective pest management.