Zetzellia mali (Ewing) is a common natural enemy of phytophagous mites, especially spider mites (Tetranychidae) and eriophyids (Eriophyidae). The biology of this mite was studied in apple orchards of Karaj, Iran, during 1999–2000. Field observations indicated that Z. mali produced three generations per year. Adult females overwintered under the bark of branches and in spurs and buds. Activity and oviposition of adults began in mid- April, and developmental stages were completed and generations overlapped during the season. The generations appeared in late April to late May, in mid-June to mid-July and in late July to September. Under laboratory conditions at 21±1°C, 65±5% RH and a 16L:8D photoperiod, a generation from the egg to adult required 20.8 days. The average preoviposition and oviposition periods were 4.6±0.1 and 11.7±2.1 days, respectively. Adult females laid one egg per day and survived without any other food on leaf discs for 10 days.