Abstract The relative change in optical density (orientational turbidimetric effect) of dilute suspensions of bacterial cells is studied theoretically and experimentally. Optical properties of bacterial cells are modeled by homogeneous spheroids using the T-matrix method. Calculated differential spectra of the steady-state turbidimetric effect, as well as its electric field dependence at fixed wavelengths, are shown to be in good agreement with experimental measurements for dilute suspensions of Escherichia coli and Bacillus polymyxa cells. The theoretically predicted unusual non-exponential decay of optical density (after removal of the orienting field) has been illustrated by an experimental electrooptical example. By using the Rayleigh–Debye–Gans approximation, we have also computed orientational effects for dilute suspensions of 3D-lattice ballistic and diffusion-limited fractal aggregates built from optically soft small spheres. We have found that the turbidimetric effect is determined solely by the gyration radius diffraction parameter and is essentially independent of the cluster mass. In terms of orientational effects, the effective structural anisotropy of the random fractal aggregates is close to the anisotropy of the homogeneous spheroidal model with the axial ratio 2 for small clusters and reduces to 1.5 for larger clusters.