AbstractUnder adjusted experimental conditions, open‐to‐air cultures of lux gene‐engineered Ralstonia eutropha (wholecell biosensors of copper) exhibit bioconvection, which accounts for fluctuating bioluminescence. The power spectrum of bioluminescence intensity fluctuations recorded from a cylindrical sample 9 mm in diameter and ∼10 mm in height is characterized by a dominant low‐frequency oscillation (with a characteristic period of ∼8–12 min), which is occasionally accompanied by a few weaker oscillations. The corresponding spectral peaks emerge on a high‐noise background. The spectra of bioluminescence intensity fluctuations qualitatively resemble the spectra of temperature or fluid velocity fluctuations in an appropriate turbulent thermal convection system. It has been suggested that in a bioconvective system, like in thermal convection systems, the emergence of oscillation reflects the large‐scale convective circulation that spans the height of the cylindrical cell. The velocity of large‐scale bioconvective circulation was estimated to be 37–48 µm/s. The occasional emergence of weaker‐than‐dominant oscillations was explained through the coexistence and interaction of the large‐scale circulation with, presumably, a gene‐expression‐related cyclic process (with a characteristic period of ∼25–50 min). Copyright © 2006 John Wiley & Sons, Ltd.