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This work reports on the existence of deterministic nonlinear dynamics in a free-running multimode semiconductor laser. The main dynamical effects found in a set of experiments realized with several edge-emitting lasers are: (i) periodic fluctuations, in the MHz range, of each modal output, (ii) compensation in the total output which remains practically flat, (iii) the modal switching sequence follows the modal optical frequencies sequence from blue to red; when the reddest mode switches off, the sequence restarts from the bluest mode. These effects are robust against a change of the control parameters which mostly affects the number of oscillating modes. Experimentally, the pump current of the lasers was increased and it was verified that the property of regular modal switching with flat total output is independent of the mode number, up to seven oscillating modes. For large number of modes, the fluctuations persist but they can lose regularity in phase and amplitude. The authors have always noticed the almost complete extinction (more than -40 dB) of the modal switching frequency in the total intensity and the switching sequence that progresses from the blue side of the optical spectrum to the red side. The frequency of the modal switching tends to increase with pumping current and with the number of active modes but it remains in the range 5 to 15 MHz. No significant difference between the power spectra of the different modal amplitudes was observed.