Carbon nanotubes and carbon nanospheres have been selectively synthesized in a polypropylene matrix by simply altering the catalyst precursor. When a mixture of ferrocene and cobalt (II) acetate was added to the polypropylene, well-dispersed carbon nanotubes were produced. When only cobalt (II) acetate was added, however, carbon nanospheres were obtained. In a third case, when the added catalyst precursor was ferrocene, no graphitic carbon was formed.