Abstract Ewes, housed in late November and lambing in late February and March, were divided into three groups of eight, six and eight animals, and received a single treatment with ivermectin (approximately 200 μg kg −1) in late February (Group I), on 15 April (Group II), or on 1 May (Group III), respectively. The egg output of the ewes and, subsequently, of eight lambs (Group IV) was measured at 2-weekly intervals by counting and identifying the larvae after culture. Group I showed a small rise in larva output in March, with counts <100 larvae per gram of faeces (l.p.g.) whereas the other groups showed considerable increase with counts >500 l.p.g. Treatment of Groups II and III just before turn out in April and May reduced the output to zero for at least 6 wk. The lambs did not show any evidence of trichostrongylid infection until the middle of July. The major parasite identified was Haemonchus contortus. The results of this trial demonstrate that 1) ivermectin treatment in February reduced but did not completely eliminate a spring rise in egg output and 2) treatment in mid-April eliminated further egg output. Ewes treated and turned out in mid-April or later were not a significant cause of pasture contamination in spring or early summer.