ABSTRACT: A population of filamentous sulfur bacteria Thioploca spp. living in the Bay of Concepción, Chile, and the adjoining shelf area was sampled for 14 mo at 4 to 6 wk intervals to investigate the influence of seasonal variations in upwellingintensity and oxygen concentrations on the population dynamics. The Thioploca population was described by its biomass, total number and diameter of sheaths, number of trichomes and species per sheath, and abundance and depth distribution ofdifferent morphological forms, e.g. trichome diameters and ratios of cell-length to diameter. Throughout the summer of 1996, oxygen concentrations in the bottom water were near zero, nitrate was 10 to 20 µM and the biomass was high, up to 160 gm^-2 wet weight without sheaths. During winter, the biomass declined due to higher oxygen concentrations under reduced upwelling intensity. The depth distribution of Thioploca spp. changed strongly with seasonal variations, but thepopulation structure remained mainly unchanged. During the ŒEl Niño¹ event in 1998, with high oxygen and low primary production, the biomass was very low. In the Bay of Concepción 2 populations of filamentous sulfur bacteria were observed, filaments withshort cells in sheaths, populating the upper 7 cm of the sediment, and filaments without sheaths living at the sediment surface.