Formaldehyde residues in muscle tissue were determined for Nile tilapia Tilapia niloticus which had been treated indefinitely with formalin at rates of 25 mg·I·1, and bath treatments of 125mg·l·1 for 1.5 h or 250 mg-1-1 for 1 h. In the indefinite treatment, formaldehyde in treated fish in­creased slightly from 48 to 72 h, and levels were back to normal at 96 h. There were, however, no statistical differences in the formaldehyde residue between formalin indefinitely treated fish and control fish. In the bath treatments, there were no significant differences in muscle formalde­hyde residue between treated and control fish, and between fish treated with 125 and 250 mg·l·1 formalin. Formalin degraded more quickly at high temperature than at low temperature. No effect of salinity on formalin degradation was found in water at 25 ± 1°c.