Formaldehyde is present in several industrial wastewaters including petrochemical wastes. in this study, the toxicity and degradability of formaldehyde in anaerobic systems were investigated. Formaldehyde showed severe toxicity to an acetate enrichment methanogenic culture. As tow as 10 mg/L (0.33 mM) of formaldehyde in the reactor completely inhibited acetate utilization. Formaldehyde however, was, degraded while acetate utilization was inhibited. Degradation of formaldehyde (initial concentration less than or equal to 30 mg/L) followed Monod model with a rate constant, k, of 0.35-0.46 d(-1). At higher initial concentrations (greater than or equal to 60 mg/L), formaldehyde degradation was inhibited and partial degradation was possible. The initial formaldehyde to biomass ratio, S-0/X-0, was useful to predict the degradation potential of high formaldehyde concentrations in batch systems. When S-0/X-0 less than or equal to 0.1, formaldehyde was completely degraded with initial concentration of up to 95 mg/L; when S-0/X-0 greater than or equal to 0.29, formaldehyde at higher than 60 mg/L was only partially degraded. The inhibition of formaldehyde degradation in batch systems could be avoided by repeated additions of low concentrations of formaldehyde (up to 30 mg/L). Chemostats (14-day retention time) showed degradation of 74 mg/L-d (1110 mg/L) of influent formaldehyde with a removal capacity of 164 mg/g VSS-day. A spike of 30 mg/L (final concentration in the chemostat) formaldehyde to the chemostat caused only a small increase in effluent acetate concentration for 3 days. But a spike of 60 mg/L (final concentration in the chemostat) formaldehyde to the chemostat resulted in a dramatic increase in acetate concentration in the effluent. The results also showed that the acetate enrichment culture was not acclimated to formaldehyde even after 226 days.