Guido Boffetta Università di Torino We study the spatial distribution of gyrotactic motile microorganisms transported by a three-dimensional turbulent flow generated by direct numerical simulations. We find that gyrotaxis combines with turbulent fluctuations to produce small scales clustering. We explain this result by showing that gyrotactic swimming cells behave like tracers transported by an effective compressible flow. The compressibility is explicitly derived in the limits of fluid acceleration much larger and smaller than the gravity.