This paper describes a direct determination of the light energy accumulation by pigmented Serratia marcescens using a photomicrocalorimetric method. At 5400 lux the rate of energy storage by the pigmented strain 9986 was 1.5 J h(-1) (g dry wt)(-1). The stored energy accumulation by pigmented strain 33 and by its non-pigmented equivalent, and also by strain 24-5 (control), was in the range 0.4-0.77 J h(-1) (g dry wt)(-1). Growing cultures contain monomer and dimer forms of the photosensitive pigment, prodigiosin. The visible light directly affected the pigment itself and provoked its phototransformation.