Photographs of groups of threespine sticklebacks (Gasterosteus aculeatus) and mummichog (Fundulus heteroclitus) from above and from the side, and comparison of regularity of spacing at various densities with that in "schools" of dead fish arranged randomly or regularly, showed that live fish were usually spaced regularly when the average distance to nearest neighbour was less than 1 fish length. At greater distances (lower densities) spacing between fish became irregular. The lowest density at which regularity occurs could be considered the critical density separating potentially regularly spaced schools of fish from loose aggregations.