The egg production of Toxocara vitulorum in Asian buffalo has been studied. Eggs were first present in the faeces of calves when they were 22.3 +/- 1.6 days old. In calves treated with pyrantel when 3 days old, the age at first patency was extended by 3.5 days indicating that there was no pre-natal transmission. Calves on only half the milk of the cow had a significantly longer prepatent period of 27.7 +/- 2.2 days. The peak egg output occurred in calves 35.7 +/- 2.6 days old and had a duration of 5.5 +/- 2.5 days with 98,000 +/- 63,700 eggs g-1 of faeces. The duration of the patent period was 35 +/- 12 days. The average lengths of populations of mature female parasites from different hosts at the time of peak egg output or older, ranged from 15.0 to 31.0 cm and was correlated with those of the males in the same populations (10.6-20.4 cm). The size of females was not affected by intraspecific competition. The proportion of males in the populations was 0.39 +/- 0.11. The egg output per female per day at the peak was 110,000 +/- 58,000 and was correlated with the size of the females at autopsy, but the egg output per female per day at the time of autopsy was lower and was not correlated, so it was concluded that the drop in egg counts was the result of reduced fecundity. The fertility of the eggs from faeces was greater than 92% throughout.(ABSTRACT TRUNCATED AT 250 WORDS)