Information on the number of clinical cases of bovine babesiosis in Northern Ireland was gathered from three sources: (a) a representative sample of farms; (b) all veterinary practices normally attending cattle; (c) the drug companies which produce babesicide. From (a) an estimate of 4520 clinical cases per year was derived, from (b) 3923 and from (c) 5600. A resultant estimate of at least 4000 cases per year was assumed to be reasonable. The mortality rate overall was 17.4 per cent but ranged from 8.8 per cent to 100 per cent on one farm. A final estimate of 12.5 per cent mortality was assumed resulting in an annual number of 500 deaths. Costs for mortality were estimated for each type of animal involved and figure of pounds 203,500 per year attributed. When the costs of treatment, assessed at pounds 31,000 per annum, unquantified production losses and increased labour costs were added an annual figure of pounds 250,000 per annum was estimated. This cost mainly falls to farmers in the three counties of Armagh, Fermanagh and Tyrone where 84 per cent of clinical cases take place. As a result the farmers in that area have to bear a loss of pounds 212,500 per year because of babesiosis.