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Nonrandom assemblies of small mammals conform, on average, to a simple pattern discovered by Barry Fox: ’’species entering a community will tend to be drawn from a different group until each group is represented, and then the rule repeats.’’ We demonstrate that the Fox assembly pattern is a probabilistic consequence of adding guild structure to models of consumer-resource competition. Assemblies agreeing with the Fox pattern will occur more frequently than those that do not agree with it. High-probability states of coexistence occurring in heterogeneous habitats will frequently include lower-probability states in habitat subsamples. In communities composed of many guilds, low-probability ’’non-Fox’’ states representing i guilds should occupy a broader range of habitat variation than should subsets of high-probability Fox states representing fewer guilds. Differences in resource gradients among habitats should produce a correlated habitat bias in guild representation. Patterns of community assembly from diverse boreal rodent communities containing four different guilds were consistent with the theory’s predictions. First, states agreeing with the Fox pattern occurred more frequently than expected to occur by chance. Second, low-probability states were nested in high-probability ones. Third, low-probability states from three guilds occupied a greater range of habitat variation than did subsets of high-probability states in fewer guilds. Finally, different rodent guilds were biased toward forest and cutover habitats.