Summary • Populations of cyclic forest insects are known to exhibit large-scale spatially synchronous dynamics. Region-wide climatic disturbances, the Moran effect, have often been invoked as the mechanism underlying this synchrony. However, no previous study has been designed specifically to examine spatial population dynamics more closely and to evaluate underlying mechanisms. • We perform a study on winter moth populations in sub-Arctic birch forest, where habitat discontinuity was included in the study design, to evaluate if dispersal barriers would act to decouple dynamics. All the study populations were situated in the same climatic region where 10-year population cycles have been known to prevail. The total spatial extent of the study was smaller than the spatial scale of synchronous dynamics reported from previous studies on cyclic insects. • We found that moth populations had mainly synchronous dynamics within sites consisting of 1·8 km transects in prime moth habitats (mature birch forest) and within a large island with continuous birch forest. However, sites on different islands could be maximally out of phase. • These results suggest that climate was not synchronizing winter moth populations in this ecosystem and that biotic mechanisms are probably involved. • Our study highlights the value of performing studies with a particularly dedicated design for elucidating the patterns and mechanisms of spatio-temporal population dynamics. Journal of Animal Ecology (2004) 73, 1129 –1136