This paper discusses the use of ‘by’-phrases in Impersonal Passives in Icelandic. It has been claimed in the literature on Icelandic syntax that ‘by’-phrases that express the agent are not very good or even ungrammatical in Impersonal Passives. The pa-per shows that this point of view oversimplifies the facts because various examples of this pattern can be found in natural data and these examples do not seem to reflect mistakes in linguistic performance. We discuss examples from the Icelandic treebank and from the web and we suggest that ‘by’-phrases are more likely to be used in Impersonal Passives if they involve new information and/or if they are heavy. One of the conclusions of the article is that large and well annotated corpora are important for linguistic research that focuses on rare constructions.