The light climate within wet and dry samples of ten mosses from the maritime Antarctic was determined using a computer-controlled optical microprobe system. Irradiances decreased with increasing depth within the moss. The rate of attenuation varied greatly between replicates reflecting the heterogeneity of the moss macro-structure. Attenuation maxima were observed at wavelengths corresponding to the peaks of chlorophyll absorption around 675 nm and below 450 nm, although the inter-wavelength differences were small compared to the inter-replicate variation. Wide inter-specific variations in light penetration were observed. These were dependent upon a number of factors, of which the most important appeared to be stem orientation, with stem density, leaf size and orientation and pigment content possibly also affecting light absorption. In most mosses the inter-wavelength variation was lost and the depth of light penetration was increased on drying. The degree to which light penetration changed on dehydration was dependent upon the relative effects of structural and pigment changes. It is suggested that, as drying occurs, the increased penetration of light into the moss increases the photosynthetic potential of deeper layers, and hence reduces the loss of productivity by the moss.