The effect of maturity stage, expressed in terms of moisture content (54,5 ± 3,2%; 66,0 ± 5,2%; and 76,3 ± 1,8%), and the effect of different rates of formalin treatment (0; 2,57 ± 0,64; 5,10 ± 0,89; and 7,59 ± 1,46 g formaldehyde/100 g crude protein) on the fermentation characteristics, in vitro dry matter digestibility, chemical composition and protein properties of maize silage were investigated. Formalin treatments were performed with and without the addition of 1 1 formic acid per ton wet material ensiled. Results in general indicate that fermentation characteristics (pH, lactic acid, total volatile fatty acid, acetic acid and butyric acid content), chemical composition (crude protein, acid-detergent fibre and water soluble carbohydrates), and in vitro dry matter digestibility were improved as the moisture content at ensiling was decreased. Formalin treatment up to a rate of 5,10 ± 0,89% led to a deterioration in fermentation characteristics and chemical composition, with a concomitant decrease in in vitro dry matter digestibility. Treatment at a rate of 7,59 ± 1,46% on the other hand, improved fermentation characteristics with no deterioration in chemical composition. Formic acid exerted no additional effect on the fermentation characteristics and chemical composition of formalin-treated silages. Protein properties, viz. true protein content and in vitro ammonia nitrogen released, were favoured while percentage acid-detergent insoluble nitrogen and the ratio of lysine to total essential amino acids were deleteriously affected by increasing rates of formalin treatment. The results in this investigation suggest an increased feeding value of maize silage ensiled with formalin at a rate of 7,59 ± 1,46%. However, provision should be made for possible harmful effects of formaldehyde on protein availability, rumen micro-organisms and the ratio of lysine to total essential amino acids.