Aggregation in sterically stabilized Fe3O4ferrofluids has been studied with the small‐angle neutron scattering technique. The influence on the aggregation of the particle volume fraction, varying between 0.02 and 7 vol % Fe3O4, and the influence of an applied magnetic field is examined. Contrast variation enabled the scattering of particles and surfactants to be measured independently. The particle size distribution derived from the scattering is in good agreement with the field dependence of the magnetization and the optical anisotropy. From the anisotropy in the neutron scattering patterns of fluids subjected to a magnetic field of 0.10 T perpendicular to the neutron beam, it is derived that at all volume fractions below 1 vol % Fe3O4most of the particles are located in small chainlike aggregates oriented parallel to the field direction. Larger, three‐dimensional aggregates arise at higher volume fractions, both in the applied field and in zero field. The interparticle distance within an aggregate corresponds to surface‐to‐surface contact of the surfactant coatings.