In vivo growth rates of Gongrosira [probably G. incrustans (Reinsch) Schmidle], a cushion-forming fresh-water green alga (Chaetophorales) are reported for the first time. The annual radial growth rate averaged 0.42 mm in a mountain stream from northern England and was positively correlated with water temperature (r=+ 0.919, P < 005). The colonies were heavily incrusted with calcium carbonate (calcite). Photosynthetic activity probably promotes calcification between the closely packed basal filaments, but most of the precipitation appears to be abiogenic from supersaturated stream waters. Calcified Gongrosira bioherms are also described from a subtropical site in Mexico. The survival period for Gongrosira at the English site was approximately one year, and at both sites Gongrosira was a permanent and significant component of the epilithic algal flora.