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In this paper we demonstrate that an assembly rule for functional groups of Australian small mammals also accounts for the structure of soricid assemblages in mesic forests of northeastern North America. In this insectivore guild, functional groups are defined by body size. A null hypothesis of random assembly was rejected, requiring the acceptance of a deterministic alternative hypothesis, such as this assembly rule. The rule was derived empirically for a small mammal community in Australian heathland and was demonstrated to account for the structure of assemblages of small mammals from many heathland and temperate eucalypt forest sites over a range of 16-degrees of latitude in Australia. The Australian assemblages included representatives of three trophic guilds (insectivore, herbivore, granivore-omnivore), representing three separate evolutionary lines. By using functional groups, rather than individual species, the assembly rule has broader application than previous species-specific rules. The rule reflects the use of available resources and demonstrates a high probability that each successive species in an assemblage will be drawn from a different functional group until each group is represented. Where sufficient resources are available the rule repeats. We conclude that assemblages of small mammals examined in Australia and North America follow the same assembly rule as demonstrated by their highly significant departures from randomly-generated assemblages. Additional evidence from Australia, North America, and England indicates that interspecific competition is the most likely mechanism responsible for the observed non-random interactive patterns. The process producing these patterns appears to act through body size in soricid communities, with larger body size conveying a competitive advantage by improving access to higher-quality foraging microhabitats.