Maternal behaviour and lamb vigour are both considered major variables affecting lamb survival, but there are few data to describe their relative importance. To investigate the relationship between lamb vigour and maternal behaviour this study compared the behaviour of pure Scottish Blackface lambs with Mule (Bluefaced Leicester ♀. × Scottish Blackface ♂) lambs, all born indoors. Anecdotal accounts suggested a significant reduction in lamb vigour in Mule lambs, a matter also of significant practical importance given the current incease in Mule numbers. Observations on the grooming behaviour of 32 single or twin-bearing ewes, and on the general activity and udder-seeking behaviour of their lambs were made at parturition. In this study, neither ewe grooming behaviour nor general lamb activity were directly related to lamb sucking success. It was shown that, although Mule lambs had high birth weights and stood as quickly as Blackface lambs, they showed significantly fewer sucking attempts (P < 0·01) and took longer to suck successfully (28 min for Blackface v. 55 min for Mules). The inability of Mule lambs to suck successfully was due to their failure to show proper udder-directed behaviour. These results indicate the importance of considering the behaviour of both ewes and lambs as it would seem that it is not ‘poor’ maternal behaviour but inappropriate lamb behaviour that is a limiting factor in lamb survival in the Mule breed.