Germination of Kalanchoë blossfeldiana Poelln. seeds is absolutely light-requiring. Germination of one seed is the result of one out of three reactions, viz. the very low fluence response (VLFR), the low fluence response (LFR) and the high fluence response/high irradiance response. In order to demonstrate the involvement of phytochrome for both photoresponses, i.e. VLFR and LFR, action spectra for induction were determined. Fluence-response data are analyzed by means of probit analysis in order to calculate the seed population parameters, with special attention to mu, or the fluence for half-maximal induction, and B, the slope in the probit diagram. Laser light was used between 620 and 800 nanometers to analyze the VLFR. Phytochrome is responsible for both photoresponses: the VLFR action spectrum demonstrates an exponential decrease in apparent photoconversion cross-section (P(r) --> P(fr)) up to about 800 nanometers. Assuming that P(r):P(fr)-X and P(fr):P(fr)-X are the effectors for the VLFR and the LFR, respectively, we estimate an average induction threshold of about 0.003% P(r):P(fr) for the VLFR and about 1% P(fr):P(fr) for the LFR among individuals of the seed population.