Size changes, measured photographically as plan area, were followed in 8 spp. of triclad [Polycelis tenuis, P. nigra, Dugesia polychroa, D. lugubris, Planaria torva, Grenobia alpina, Polycelis felina and Dendrocoelum lacteum] during complete starvation. The area-time curves so obtained were negative exponentials. In common with simple physico-chemical reactions the relationship between reaction rate, expressed as the coefficient of exponential degrowth, and the reciprocal of ambient temperature (in degrees Kelvin) was linear. There was a broad relationship between the rate of degrowth, its sensitivity to temperature and the distribution of each species.