Periodic instabilities of frequency f0 in an electron-hole plasma in Ge are excited with an external perturbation V1 sin (2f1t). Varying V1 and f1 results in quasiperiodic, mode-locked, and chaotic states comparable to those expected theoretically. Increasing V1 while keeping the ratio f0f1 fixed at the golden mean (1.6180...), we observe a universal transition to chaos, in quantitative agreement with theory. Driving the instability simultaneously at two incommensurate frequencies, f1 and f2, we observe transitions between three-frequency quasiperiodicity, two-frequency mode locking, and chaos. © 1986 The American Physical Society.