The application of quantum field theoretical methods to strongly interacting many-body problems has reaped rich rewards. Foremost, it has nurtured the quasi-particle notion. The introduction of suitable fictitious entities permits to cast otherwise notoriously difficult 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 still capture the essential physics. A most notable recent example occurs in the conduction properties of a two-dimensional electron system, when exposed to a strong perpendicular magnetic field B. They are governed by electron–electron interactions, that bring about the Nobel prize winning fractional quantum Hall effect (FQHE) (Perspectives on Quantum Hall effects, Wiley, New York, 1996). Composite fermions (CFs), that do not experience the external magnetic field but a drastically reduced effective magnetic field B*, were identified as opposite quasi-particles that simplify enormously the understanding of the FQHE (Phys. Today (2000) 39; Phys. Rev. Lett. 63 (1989) 199). They behave as legitimate particles with well-defined charge, spin and statistics (Phys. Rev. B 47 (1993) 7312; Composite Fermions, World Scientific, Singapore, 1998; Phys. Rev. Lett. 70 (1993) 2944; 75 (1995) 3926; 71 (1993) 3846; 72 (1994) 2065; 77 (1996) 2272). They precess, like electrons, along circular orbits, with a diameter determined by B* rather than B, and with a frequency that is hard to predict, since the effective mass remains enigmatic. Ever since their prediction, the demonstration of enhanced absorption of a microwave field that resonates with the frequency of their circular motion was considered the ultimate experiment to unravel this issue. Here, we report the observation of this cyclotron resonance of CFs with two and four flux quanta and extract their effective mass.