Observations on the larvae of sheep trichostrongyles on hill pastures showed:

1. A reduction in larval populations occurred when the ‘moving-flock’ type management was used. The large proportion of eggs passed in faeces is deposited on a part of the pasture where little or no grazing occurs.

2. Local concentrations of larvae occurred on the grazed portions of a pasture when grazing was uneven. Thus, the apparent low rate of stocking on hill pastures does not give a true picture of the potential dangers of parasitic disease. Estimates of pasture contamination by Taylor's method of sampling were deceptively low.

3. The number of larvae on grass blades was lowest when the grass was wet with dew. Sampling of pastures between 12 noon and 5 p.m g.m.t. gave maximal and most constant estimates.

4. There was an increase in the number of larvae on a pasture during the warmer months of the year. While low temperatures prevent or delay hatching, the increase in population was not entirely due to the increased rate of development when temperature was high. The increase in numbers was partly due to an increase in the number of adult worms in the sheep hosts.

5. Removal of sheep for 3 weeks reduced the number of larvae on a pasture. This reduction was not apparent, however, until after the first 12 days. After 3 weeks the pasture population was reduced by 55 %, while after 4 weeks (i.e. 7 days after the return of the sheep) the reduction was 90%. Two weeks later there was a small increase in the larval population, but this was again reduced with the onset of colder conditions.