The effect of photoperiod and temperature on the biology of nymphs and adults of Piezodorus guildinii (Westwood) was studied in the laboratory. Four different conditions were tested (14:10, 12:12 and 10:14 h L:D at 25 °C, and 10:14 h L:D at 20 °C), at 80 ± 10% RH. The shortest nymph development time was recorded at 14:10 h L:D (25 °C) (21.5 days) and the longest at 10:14 h L:D (20 °C) (42 days). The highest nymph mortality rate was recorded at 10:14 h L:D (20 °C) and the lowest at 14:10 h L:D (25 °C) (84.5 vs. 24.2%). Newly emerged females reared at 14:10 h L:D (25 °C) were heavier than those of the remaining treatments. Fresh body weight gain (mg) occurred only during the 1st week of adult life. Adult survivorship was highest at 10:14 h L:D (20 °C) and lowest at 14:10 h L:D (25 °C). Total longevity was shortest when adults were held at 14:10 h L:D (25 °C) and longest at 10:14 h L:D (20 °C) (38.6 vs. 98.8 days). The maximum percentage of ovipositing females occurred at 14:10 h L:D (25 °C) and the minimum at 10:14 h L:D (20 °C). Females maintained at 14:10 h L:D (25 °C) and 12:12 h L:D (25 °C) produced similar and greater number of egg masses than females at 10:14 h L:D (25 °C) and 10:14 h L:D (20 °C). The number of eggs/female was the greatest at 14:10 h L:D (25 °C) and the lowest at 10:14 h L:D (20 °C) (196.2 vs. 21.7 eggs/♀). Egg viability was similar under different photophases at 25 °C, while significantly reduced at 10:14 h L:D (20 °C) (54 vs. 4.1%). The longest egg incubation period was recorded at 10:14 h L:D (20 °C) and the shortest at 14:10 h L:D (25 °C) (7 vs. 4.1 days). These laboratory results suggest that P. guildinii does not reproduce during the time its preferred host soybean is unavailable at latitude 30°–35° S, which corresponds approximately to the conditions tested at 10:14 h L:D (20 °C).