Aggression and territory size of the farmer damselfish Plectroglyphidodon lacrymatus were examined in the presence of adjacent territories of conspecific and heterospecific farmers that maintain algal turf territories on the reefs of Anda, north‐western Philippines. Plectroglyphidodon lacrymatus territories with no adjacent neighbours had a mean size of 1.2 m2 whereas those adjacent to conspecifics were 47% smaller. In contrast, the mean size of territories adjacent to a heterospecific farmer (i.e., Pomacentrus burroughi, Pomacentrus chrysurus) did not differ from that of solitary individuals. Aggression towards conspecific farmers (84% probability) was higher than heterospecific farmers (31%). The higher aggression towards conspecific farmers suggests that the energetic costs for defence increased, which may have caused a decrease in territory sizes. Despite the higher aggression towards conspecific farmers, the thickness and percentage cover of algae inside the territories of focal fish did not appear to vary according to type of neighbouring farmer although algal turf cover was higher and juvenile coral occurrence was lower inside than outside the territories. Our results show that the territorial behaviour of P. lacrymatus varies with conspecific and heterospecific farmers, which may influence their distribution and benthic habitat characteristics.

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