Based on transmission electron microscopy observation, zoosporogenesis in the siphonous multinucleate green alga Derbesia tenuissima can be divided into five stages. In vegetative sporophytes and Stage 1 sporangia, the cytoplasm was distinguished as two regions, the inner region of chloroplasts and the outer region of nuclei. In Stage 1 sporangia, the chloroplasts differentiated into amyloplasts and contained large starch grains. In Stage 2 sporangia, several meiotic nuclei with synaptonemal complexes were observed in the apical and the subapical portions of a sporangium. The width of a synaptonemal complex was 103′5 (mean ′ standard deviation) nm (n = 20). In Stage 3 sporangia, the plug complex that was composed of 4 layers as observed with transmission electron microscopy, was formed between the sporangium and the main axis. In Stage 4 sporangia, the following conspicuous characters were observed: 1) differentiation of nuclei into two types, 2) de novo appearance of centrioles (flagellar basal bodies) on the type 2 nuclei (large), 3) decrease in the number of the type 1 nuclei (small), 4) development of cleavage furrows and vesicles, 5) formation and elongation of flagellar axonemes, and 6) development of three characteristic bands on the flagellar basal bodies. In Stage 5 sporangia, differentiation of flagella and flagellar bands was completed. Each stephanokont zoospore of D. tenuissima had about 80 flagella, about 16 nuclei and many chloroplasts.