We have imaged Langmuir-Blodgett (LB) films of cytochrome f (Cyt f), transferred onto a gold film on mica substrate which has been covered with arachidic acid (three monolayers), with the scanning tunneling microscope. From these images, we have determined the structure of the Cyt f LB films (two monolayers) as a function of surface pressure (II) and ethanol concentration (ethanol:water) in the spreading solvent. We have observed aggregation of the Cyt f protein in films deposited at 15 and 20 mN/m with ethanol concentrations of 20%, 40%, and 80% in the spreading solvent. The LB films of Cyt f are mainly composed of aggregates a few hundred nm in size. Film morphology and aggregate sizes depend upon the ethanol concentration and the surface pressure for transfer onto the solid substrate.