The importance of correlations between soil particle composition and the physical characteristics of Sand Martin Riparia riparia breeding holes has received very little attention. I used dry sieve analysis and decantation to examine 14 particle size ranges of samples from 654 breeding holes collected at 106 breeding colonies in the Czech Republic and at three in Great Britain. All five measured physical characteristics (tunnel depth, width and height of the entrance opening, slope of the tunnel and the distance to the bank top) were significantly correlated with measured soil particle size ranges. Tunnel depth, which is one of the most important factors influencing breeding success in this species, increased as the proportion of small particles (<900µm) in the soil increased.