The cause of ill thrift and deaths was investigated in a group of 40 Scottish lowground ewes, which had been treated with levamisole and then turned onto clean grazing after lambing, 6-8 weeks previously. Concurrent haemonchosis and teladorsagiosis were diagnosed, putatively associated with the spring maturation of large numbers of hypobiotic larvae. Ill thrift due to parasitic gastroenteritis consequently occurred in the lambs which had been turned onto the 'clean' grazing with their dams, despite metaphylactic anthelmintic treatments. The contributory role of haemonchosis resulting in serious production-limiting disease is unusual in Scottish sheep flocks. It is suggested that the problem may have arisen as a consequence of the adaptation of Haemonchus contortus to survive overwinter in a cold climate as arrested early fourth stage larvae, which were not eliminated when the ewes were treated with levamisole at turnout. (c) 2007 Elsevier B.V. All rights reserved.