All-female triploids were produced by heat shocking 2.5-min-old eggs which had been fertilized with sperm from a masculinized female (XX-♂). Induction of triploidy was confirmed using the estimates of red blood cell (RBC) nuclear volume. Ovaries of the control diploids were packed with oocytes, while those of triploids contained only one or two poorly developed oocytes, thus indicating the sterility of female triploids. The major disadvantage of normal triploidization is the inadvertent introduction of a few diploids (♀ + ♂), which can undo all the labour involved in the triploidization; but all-female triploidization avoids the problems associated with normal triploidization. This technique may have considerable potential for tilapia aquaculture.