Summary The germination of Citrullus lanatus seeds is inhibited by far-red-, red-, blue-, green- and white light. The inhibitory effect of light (with the exception of far-red) light is alleviated by a follow-up dark treatment. In addition it was found that the inhibitory effect of far-red light could not only be alleviated by a follow up red light treatment but also by short subsequent exposure to white- and blue light. The obtained results most probably indicate that germination is inhibited by red, blue, white, and, to a lesser extent, by green light due to pigment cycling between P r and P fr. During periods of pigment cycling much of the phytochrome probably accumulates in a form which can readily change to P fr in a follow up dark treatment. It appears that due to inherent differences in the P fr-threshold value necessary for germination, negatively- and positively photoblastic seeds cannot be made the same in their light requirements for germination merely by manipulating the phytochrome concentrations with light.