An abattoir survey was carried out during the 1991 to 1992 slaughter season to examine the structure of the ostertagid populations in red deer. Most of the animals were 15- to 22-month-old stags, but there were also hinds on two occasions and calves on one. Ostertagids of mixed species were present in all of the animals, and 43 of 120 (35.8 per cent) stags and 10 of 17 (58.8 per cent) hinds, but no calves, also carried small numbers of Trichostrongylus axei. Most animals carried adult and developing ostertagids and after September they all contained hypobiotic larvae (EL4). EL4 were present in three of nine stags in September and they increased in number through the autumn. In stags killed between November and February the group mean numbers of EL4 represented 62 to 94 per cent of the total burdens present, compared with 37 and 55 per cent on April 1 and 7, respectively. In the hinds, the number and the structure of the ostertagid populations was similar to that in the stags. Of 14 calves killed on October 9, only two contained a few EL4. Subcutaneous ivermectin at 400 μg/kg removed 100 per cent of the adult and developing worms, but although it also removed 95 per cent of EL4, some larvae remained in every treated animal. The efficacy of fenbendazole at 15 mg/kg given over five days was 75 per cent against EL4 and although the long interval after treatment obscured the result, it was probably much higher against later stages.