SummaryThe decomposition of grasses and rye labelled with14C was studied using ground material and also fragments cut from intact leaves or roots either placed on the soil surface or buried in the soil incubated under various conditions. Autoradiography was used to observe the changes in the decaying plant tissue with a minimum of disturbance. Autoradiograms prepared before incubation and subsequently at intervals reveal an over‐all fall in density of the images, a complete disappearance of14C in small discrete sites, as well as a dispersion of14C over distances of several cm from the plant residues.A photoelectric technique was devised by which changes in density could be expressed quantitatively. The log density of autoradiographic images of pellets of ground grasses shows a predominantly, though not completely, linear regression on time of incubation. The method has shown that the process of decomposition is very slow, that in the first stages of decomposition ryegrass decays faster than cocksfoot, and that ground material tends to behave in a different manner to fragments cut from intact tissues. Changes in the area of autoradiographic images with time of incubation could be used as an additional but less sensitive measure of the rate of decomposition.The participation of micro‐organisms (especially fungi) in the breakdown of the plant tissues has been demonstrated by the presence of labelled organisms in the vicinity of plant residues. Labelled fungi are more numerous in the first 3 months of incubation, during which a marked fall in image density of the plant residues occurs. A further decrease in image density is frequently associated with the appearance of fungal resting structures with a greater concentration of14C than the surrounding plant fragments. Because of their longevity these structures contribute to the fixing of14C in a different fraction of the biomass. Faecal pellets of soil mesofauna also concentrate14C and resist decomposition for very long periods of t