A laboratory study was carried out to determine the breakdown of soil structure by alternate freezing and thawing. Surface samples of Haldimand clay from plots cropped with corn for the 3 previous years and from plots of continuous blue grass sod were used.Cycles of freezing and thawing caused a significant breakdown of aggregates from the sod plots at all moisture levels employed except the one equivalent to 1.0 atmosphere pressure. The destructive effects of alternate freezing and thawing increased with increasing moisture in the well-aggregated soil (blue grass), but the effects on a poorly aggregated soil (corn) were not significant.