The freshwater prawn Caridina weberi was found to show a diurnal rhythm of colour change when placed on a specially devised' neutral' background in natural light. The red and white pigments were dispersed during day time and fully concentrated at night. A daily cycle of chromatophorotropin contents was noticed in eyestalk, supraoesophageal ganglia and circum-oesophageal commissures kept in such a condition. Among these, the red pigment dispersing hormone (RPDH) of supraoesophageal ganglia and the white pigment concentrating hormone (WPCH) of circumoesophageal commissures were more in amount during day time while the RPDH of eyestalk was more in the night time. The red pigment concentrating hormone of various nervous tissues changed little in amount during the 24-hour cycle.