Carbon black (CB) filled polymers display mechanical and electrical properties that depend not only on the volume fraction phi of the filler particles but also on their structure and on their distribution. Recently it has been shown that ultra-small-angle X-ray scattering measurements (USAXS) yield information on the arrangement of aggregates in the fluffy or pelletized CB powder and in particular on their mutual interpenetration. In this paper, USAXS measurements using synchrotron radiation and a two-dimensional detector are reported. They reveal anisotropic features in carbon black filled polymers having a length scale around 100 nm. For CB concentrations below the percolation threshold, no anisotropy is observed, even in uniaxially stretched samples. For concentrations above the threshold, however, the results are highly sensitive to the degree of interpenetration of the CB aggregates, The role of aggregate interpenetration (which can occur because the fractal dimension is smaller than 2) in the mechanical behavior of CB-polymer composites is shown. These features are fully consistent with recent models of reinforcement of rubber by fractal aggregates.