Abstract The potential value of fish silage as a protein source for ruminant stock was assessed by chemical analysis and by incubation with rumen fluid in vitro. Six fish silages were prepared from a mixture of whole white fish and fish offals. Formic acid (90%) was added to each silage after mincing (20 ml kg −1). Silage F0 received only formic acid while silages F1, F2, F3, F4 and F5 received in addition 1, 2, 3, 4 and 5 ml kg −1 of formalin solution (40% HCHO), respectively. Liquefaction occurred after storage for three days at 23°C. All silages could be handled as slurries, but viscosity was increased by formaldehyde treatment. Formaldehyde addition to the fish waste increased the proportion of protein precipitatable by trichloroacetic acid (g kg −1 CP) from 133 to 416 for silages F0 and F5, respectively. Formaldehyde addition also reduced the rate of ammonia release (kNH 3 % h −1) from fish silage protein measured in vitro over five hours of incubation with rumen fluid. Silage F0 gave a similar value for kNH 3 to that measured for casein (6.1 and 5.8, respectively). For silage F5, kNH 3 was 4.3, which was significantly lower than F0 ( P < 0.01) but still much higher than a value of 1.6 measured for white fish meal. Incubation with urea gave very high rates of ammonia production in the first hour. Addition of 10 ml kg −1 formalin to fish waste produced a rubber-like product which was considered unsuitable for feeding. It was concluded that treatment of acid-preserved fish waste with 5 ml kg −1 formalin has potential for the improvement of its protein value for ruminants and gives an easily handled product for incorporation into diets.