The introduction of suitable fictitious entities occasionally permits to cast otherwise difficult strongly interacting many-body systems in a single particle form. We can then take the customary physical approach, using concepts and representations which formerly could only be applied to systems with weak interactions, and yet still capture the essential physics. A most notable recent example occurs in the conduction properties of a two-dimensional electron system (2DES), when exposed to a strong perpendicular magnetic field B. They are governed by electron-electron interactions, that bring about the fractional quantum hall effect (FQHE). S. Das Sarma and A. Pinczuk (eds.), Perspectives on Quantum Hall Effects (Wiley, New York, 1996). Composite fermions, that do not experience the external magnetic field but a drastically reduced effective magnetic field B*, were identified as apposite quasi-particles that simplify our understanding of the FQHE. J. K. Jain, Phys. Today, 39-45 (2000). J. K. Jain, Phys. Rev. Lett. 63, 199-202 (1989). They precess, like electrons, along circular cyclotron orbits, with a diameter determined by B* rather than B. B. I. Halperin, P. A. Lee, and N. Read, Phys. Rev. B 47, 7312-7343 (1993). O. Heinonen, (ed.), Composite Fermions (World Scientific, Singapore, 1998). R. R. Du, H. L. Stormer, D. C. Tsui, L. N. Pfeiffer, and K. W. West, Phys. Rev. Lett. 70, 2944-2947 (1993). R. R. Du et al., Phys. Rev. Lett. 75, 3926-3929 (1995). R. L. Willett, R. R. Ruel, K. W. West, and L. N. Pfeiffer, Phys. Rev. Lett. 71, 3846-3849 (1993). V. J. Goldman, B. Su, and J. K. Jain, Phys. Rev. Lett. 72, 2065-2068 (1994). J. H. Smet, D. Weiss, R. H. Blick, G. Lutjering, and K. von Klitzing, Phys. Rev. Lett. 77, 2272-2275 (1996). The frequency of their cyclotron motion remained hitherto enigmatic, since the effective mass is no longer related to the band mass of the original electrons and is entirely generated from electron-electron interactions. Here, we demonstrate the enhanced absorption of a microwave field that resonates with the frequency of their circular motion. From this cyclotron resonance, we derive a composite fermion effective mass that varies from 0.7 to 1.2 times the electron mass in vacuum as their density is tuned from 0.6 x 10(11)/cm(2) to 1.2 x 10(11)/cm(2).