Literature relating to the effects of the tapewormMoniezia expansaon sheep is reviewed. While there are many claims of adverse effects by this organism, none of these withstand critical scrutiny. The balance of evidence indicates thatM. expansainfections are generally harmless, even when the tapeworms are present in large numbers in young lambs, and that there is no justification for treating sheep forM. expansaon the basis of any likely benefit to the health or production of the animals. No published experimental results can be found to support claims that heavyM. expansainfections sometimes exacerbate losses from other causes.