The most frequent form of social interaction between pairs of wild or domestic rats (Rattus norvegicus) was passing behaviour in the narrow burrows and open field of seminatural environments. This behaviour occurs when a moving rat approaches another, either head-to-head or head-to-tail; the passed animal freezes while the passing rat continues, side touching side. The social dominance manifest by this behaviour correlated significantly with fighting dominance, particularly among female wild and domestic rats. In addition, dominance during displacements for access to food and water correlated with the dominance relationship in passing within pairs of domestic female rats. Although both passing and general activity had the same crepuscular daily rhythm, passing dominance did not correlate with individual differences in general activity level. Thus passing indicates social order and is not simply a by-product of general activity. Because passing occurs ten times more frequently than fights, in future studies it may be a more powerful indicator of female dominance and social order.