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Abstract: Liometopum microcephalum (Panzer, 1798) is a rare arboricolous ant, which forms large colonies of high ecological importance and ranks at the top position in the hierarchy of ant assemblages. Many aspects of the species' biology remain unknown due to its scattered occurrence and bad nest accessibility. Published information is sometimes inconsistent, such as in the case of worker polymorphism. Our objectives were (1) to determine the level of polymorphism, (2) to ascertain if workers occupied by different tasks differ in size, and (3) to assess the effect of competitors, habitat type and territory size (as a proxy for colony size) on worker size. Fifteen colonies, with or without a competing ant species in their vicinity, were sampled in spring and summer 2011 in the south-eastern part of the Czech Republic. Head width, head length, hind femur and tibia lengths were measured as indices of worker size. Territory areas, assessed during each sampling, served as an indicator of the size and vitality of individual colonies. Worker size variability was continuous, with a broad size range for all measured characters within each studied colony. We found different levels of polymorphism for measured body parts in individual colonies: isometry and simple, diphasic and triphasic allometry; in most colonies, the level changed in the course of time. Generally, workers collected in spring were larger than those collected in summer (p < 0.0001). We did not find any differences between workers performing different tasks outside the nest. We found a positive correlation between territory size and body size, represented by mean femur length (p = 0.0068). Territory size was affected by the presence of behaviourally dominant ant species (p = 0.0036), in particular Lasius fuliginosus (Latreille, 1798). We conclude that in contrast to information in literature the species is not truly dimorphic and even colonies seemingly made of workers of one size class contain a wide range of worker sizes.