The relationship between bacterial load and the microalga Chaetoceros muelleri was analyzed in a scale-up experiment. The microalga was grown during five days in a 0.4-L Erlenmeyer flask, 2-L Fernbach flask, 18-L Carboy and 400-L column, during which the cell density of C. muelleri, the abundance of heterotrophic bacteria, Vibrio spp., and total bacteria were determined. The highest specific growth rates (μ) of C. muelleri occurred during the first day of culture (0.88 to 2.29 d^-1). Highest cell density was recorded on the fifth day at the 2-L (7.62 × 10⁶ cells·mL^-1) and 18-L (6.32 × 10⁶ cells·mL^-1), coinciding with the maximum counts of heterotrophic bacteria (16.55 × 10⁵ and >30 × 10⁵ CFU·mL^-1, respectively). There was a high correlation (0.80, 0.75, 0.85; p <0.05) between microalgal cell density and total bacteria in the first three culture volumes and a low correlation (0.27; p = 0.34) at 400-L column. The highest mean concentration of total bacteria (884.13 × 10⁵ cells·mL^-1) during the five days occurred at 18-L Carboy. The concentration of total bacteria at all levels was always higher than that of heterotrophic bacteria. The average ratio of heterotrophic to total bacteria was higher in the 2-L (0.0108) and 18-L (0.0172) cultures. The high biomass of C. muelleri and the presence of Vibrio spp. at the 18-L and 400-L levels indicate that it is necessary to establish programs to prevent diseases and economic losses caused by pathogenic bacteria in penaeid shrimp farming.