Farmyard manure was subjected to anaerobic fermentation in airtight digesters for approximately 30 days. The annual biogas production was 0.9 m(3) per large animal unit per clay with three different production levels evident: a summer period, a transitional period and a winter period, when biogas production was 1.1, 0.75 and 0.55 m(3) per animal unit per day, respectively The research focused on the factors influencing equipment operation for farmyard manure anaerobic fermentation and biogas production. The factors discussed in this paper can be divided into three different groups: (i) main independent factors (natural conditions, transition to summer and/or winter feeding); (ii) main factors affected by the operational conditions (composition of feed, aerobic stage); and (iii) secondary factors that may nor influence fermentation under optimum operational conditions (feed quality, quantity and quality of bedding, technology of farmyard manure removal). Once the influence of secondary and partly, main controllable factors had been eliminated, higher biogas production could be expected mostly in the transitional and winter periods, up to 0.85 and 0.65 m(3) per large animal unit per day; the annual average biogas production would then reach 0.95 m(3) per large animal unit.