Abstract Clutton-Brock & Parker (1995, Animal Behaviour, 49, 1345–1365) proposed that sexual coercion is a third component of sexual selection, along with intrasexual competition for mates and intersexual mate choice. They stated that sexual coercion occurs when males subject females to forced copulation, harassment, or intimidation, and that aggression towards females may occur even when females merely associate (not mate) with other males. We tested this proposition in laboratory experiments with red-backed salamanders, Plethodon cinereus, that had been found in male–female pairs (we call members of a pair ‘partners’) and as single (nonpaired) individuals in the forest. We manipulated paired females so that they either had associated with another male (socially polyandrous partners) or had not associated with another male (socially monogamous partners) before returning to their partners' territories. We also manipulated single females so that, upon encountering a single male for the first time, they either had associated with another male (socially polyandrous strangers) or had not associated with another male (socially naı̈ve strangers). Males were significantly more aggressive towards and stayed farther away from female partners that were socially polyandrous relative to those that were socially monogamous, during both the summer noncourtship season and autumn courtship season. Socially polyandrous females increased escape behaviour relative to socially monogamous females. However, males that met females for the first time did not differ significantly in aggression towards or distance from them when those females were either socially polyandrous or naı̈ve, during both the noncourtship and courtship seasons. Thus, male aggressive behaviour was context dependent, with enhanced aggression only towards partners that were socially polyandrous. We infer that males attempted to manipulate female partners into social monogamy by increasing aggression towards socially polyandrous partners. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.