1. Breeding habitat variation in little blue penguin agonistic behaviour is correlated with environmental heterogeneity. 2. Cave-dwellers nested in tight colonial aggregations and had no physical barriers between nest sites. Burrow-dwellers nested as solitary pairs or in loose aggregations and were physically isolated from one another. 3. Cave-dwellers had significantly higher agonistic interaction rates than burrow-dwellers. 4. Cave-dwellers used a total of 22 distinct agonistic behaviours with 54 variations. Burrow-dwellers used a total of 13 agonistic behaviours with 29 variations. Variations on base behaviour were primarily the result of differences in vocal accompaniment. 5. Both cave- and burrow-dwellers used growl, low bray, medium bray, full bray, aggressive bark and aggressive yell vocalizations during agonistic behaviour. Only burrow-dwellers used a hiss vocalization. 6. Despite variation between cave- and burrow-dweller repertoires, many agonistic behaviours were very similar in form (posture, duration, movement, presence or absence of vocal components) and/or context (distance from the opponent when performing the behaviour, proportion of interactions involving the behaviour). 7. Repertoires from both habitats could be divided into three discrete categories: defensive behaviour, offensive behaviour, and overt aggression. For each habitat, the defensive behaviour category could be divided into stationary and distance increasing behaviour; the offensive behaviour category could be divided into stationary, distance reducing, and contact behaviour; and the overt aggression category could be divided into distance reducing and contact behaviour. 8. Cave-dwellers used twice as many defensive behaviours, twice as many offensive behaviours, and the same number of overtly aggressive behaviours as burrow-dwellers. 9. In both habitats, defensive behaviour was used most when the opponent was nearby ( < 1 m to 1-2 m), whereas stationary offensive behaviour was performed most when opponents were further away (2-3 m to > 3 m). Distance reducing behaviour (both offensive and overt) was performed most at middle interaction distances (1-2 m to 2-3 m) in both habitats. 10. Males were involved in a higher proportion of agonistic interactions than females in both habitats. 11. Burrow-dwellers used the overt behaviour Attack significantly more than cave-dwellers and also used the most dangerous fighting method more commonly than cave-dwellers. Burrow-dwellers also bit and fought significantly longer than cave-dwellers. 12. An egg transfer experiment (i.e. between cave and burrow colonies) indicated that chicks may not be genetically bound to the use of a habitat-specific repertoire of agonistic behaviours. 13. Four possible mechanisms are suggested for the origin of variation between habitats: (1) genetic influences; (2) phenotypic modulation; (3) experience; and (4) circumstantial influences. 14. Plastic agonistic behaviour may allow immediate and potentially adaptive phenotypic change in response to environmental heterogeneity. The large cave repertoire may reduce the chances of any one interaction ending with overt aggression. The smaller burrow-dweller repertoire may be sufficient to defend the physically enclosed burrow nest sites.