Maintenance requirements and reliability are reported, both for two identical continuously mixed digesters each of 125 m 3 capacity, operated on dairy cow slurry, and for the ancillary equipment associated with the digestion plants. Ancillary equipment is defined as any equipment which would need to be in place even if no digestion plant were installed i.e. chain scraper, reception pit and pumps. Accurate records of plant performance and maintenance requirements were kept during 3 years of operation at a farm site. One digester was operated throughout on whole slurry, while the other was operated for 70% of this time on slurry from which the solids had been separated. The number of occasions on which each digester system required maintenance was 22 per annum. For the separated slurry system this total arose from the digester itself needing attention 15 times per annum, on average, and the slurry separator seven. Annual maintenance costs were £415 and £445 for the whole and separated slurry systems, respectively. The labour requirement for maintenance was 36 and 54 h/a for the whole and separated slurry systems, respectively. The requirement for the separated slurry system was higher because of the 32 h/a required to maintain the separator itself. The costs of maintaining the slurry separator were ascribed to the separated slurry digester plant, since the separator is a pre-requisite for such a system. The single items on the digestion plants themselves, requiring attention on the greatest number of occasions were the gas boilers (12 per annum on each plant). The ancillary equipment upstream of both digestion plants (and shared by them) needed an average of 25 maintenance operations per annum, requiring 61 h of maintenance work, at a total average cost of £1088 per annum. This equipment would, of course, be required regardless of whether or not the waste was subsequently treated by anaerobic digestion. This equipment is thus shown to be less reliable than either the whole or the separated slurry digester systems, and had a greater annual maintenance cost than that of the two digester systems combined. Most failures were of a mechanical type and did not require expertise specific to the anaerobic digestion process to rectify them. The historic ratios of annual maintenance costs to capital costs were 0·9% and 0·8% for the whole and separated slurry systems, respectively. Both these figures are lower than might be inferred from the literature for process plant.