The present work deals with the 17 species belonging to the subfamily Thyasinae known to occur in North America. Since there are still large areas of North America which have been poorly collected for water mites, this list is without doubt incomplete. There are several thyasin genera, with a widespread palearctic distribution, which are as yet unreported from this continent. Because so many water mite genera, formerly known only from the Old World, have been collected in North America during the last few years, it seems probable that members of some or all of these European thyasin genera may be taken in future collections.