The biology of Bryobia rubrioculus (Scheuten) (Acari:Tetranychidae) was studied on black cherry (Prunus serotina Ehrh.) trees in an orchard of Baraghan in 2003 and also under laboratory conditions at 22 +/-1 deg C, 70 +/- 5% RH and 16L: 8D photoperiod. The activity of overwintering eggs began in early April and produced 3 generations per year. In the laboratory, the mean time for development from egg to egg took 35.22 +/- 1.21 days for females. The average preoviposition, oviposition and postoviposition periods were 3.16 +/- 0.65, 13.2 +/- 0.71 and 0.8 +/- 0.27 days respectively.