The formation of the photosynthetic apparatus in the wild type Rhodopseudomonas sphaeroides and in the "Phofil" mutant was investigated by analyzing absorption and fluorescence parameters and the kinetics of fluorescence induction. Repigmentation was induced by transfer of bleached cells to semi-aerobic culture conditions. 1. In the "Phofil" mutant, functional photosynthetic units appear at pigment cellular contents which depend on the physiological state of the inoculum. The unadapted mutant can reach the functional units stage at a cellular pigment level 20 times lower than that of the wild type, although the size and composition of the units are identical in both strains. This rules out the hypothesis of photosynthetic units forming by random integration of pigments into the membrane. Moreover, the fact that units are separate at this stage (as shown by the exponential shape of fluorescence induction kinetics) suggests that the units' formation proceeds from discrete predetermined membrane sites. 2. In repigmentng wild type cells, the multistep assembly of bacteriochlorophyll complexes appears correlated to their organization in photosynthetic units as follows: (i) During a first stage, B-875 is mostly synthesized, while the efficiency of transfer increases, suggesting that the pigments are initially interpersed at comparatively large average distances from each other in the bleached membrane. (ii) After 1.5 h of repigmentation, the transfer and trapping efficiencies reach a maximum. The units (26 B-875 + 11 B-850 per center) are still separate, as shown by exponential fluorescence kinetics. (iii) The increase in the units' size then essentially proceeds through B-850 incorporation. Energy transfer between units occurs at a comparatively late stage, i.e., 5 h repigmentation.