In the cultivation of a marine diatom Chaetoceros muelleri (Lemmermann, 1898), flat plate vertical glass reactors with light path lengths of 1 or 3 cm were used in the laboratory under a constant photon flux density (PFD) of 190 μmol photon m−2 s−1. In the first experiment, equal inocula were added in the two bioreactors (1 and 3 cm light path); by the end of the second day, cell concentration was low in the 1-cm light path bioreactor, possibly indicating photoinhibition, since growth of the culture increased in the 3-cm light path bioreactor. As for in the second trial, the areal volumes were equal, that is, the cell concentration was increased three-fold in 1 cm light path to equalize the areal optic densities in both reactors. After the harvest started, average of the volumetric output rates in 1 cm (PV1 cm) and 3 cm light paths (PV3 cm) were calculated to be 0.22 and 0.10 g l−1 day−1, and areal output rates in 1 cm (PA1 cm) and 3 cm (PA3 cm) light paths were calculated to be 1.80 and 2.53 g m−2 day−1, respectively.