Sampling strategy, in terms of physical size and positioning of the sampling units, may affect strongly the results of spatial surveys. The aim of this study was to analyse the effect of sampling unit size on the perception of spatial patterns of earthworm (Lumbricus terrestris L.) middens. The implications for optimal sampling strategy for spatial interpolation were assessed. Spatial variation of midden density was investigated in Vaisakko forest, south-western Finland, using 225 sample points distributed on a square grid with a minimum distance of 25 m between samples. At each point, middens were counted within samples of sequentially increasing size (sample surface 0.125, 0.25, 1 m2) and analysed by means of geostatistics. The results showed significant spatial continuity of midden distribution in all cases. Whereas, neither the estimate of mean middens density nor the global distribution pattern were markedly affected by sample unit size, the total variance increased considerably with decreasing sample unit area. Isotropic variograms for different sample unit sizes were all spherical but large discrepancies in the model parameters were observed. The nugget variance tended to decrease with increasing sample unit size while the spatial variance increased slightly. Since changing sample unit size affected the variogram we also investigated the consequences in terms of optimal sampling strategy for spatial interpolation by punctual kriging. Increasing the quadrat size from 0.25 to 1 m2 and simultaneously increasing the sample spacing from 25 to 50 m, so that the sampling effort was constant in terms of total surface investigated, did not affect the kriging standard deviation. The positive effect of increasing quadrat size was thus enough to compensate the negative effect of the correspondingly sparser sampling grid. The results showed that while the sampling unit size did not have strong effect on the perception of general midden distribution in the forest, it did have marked consequences in the spatial modelling of the phenomenon. The issue of sampling unit size is clearly worthy of careful consideration in the planning of field studies and geostatistical tools can be put to good use in evaluating the pros and cons of different sampling strategies< Réduire