Rapid progress in passively mode-locked fibre lasers 1-6 is currently driven by the recent discovery of new mode-locking mechanisms, namely, the self-similarly evolving pulse (similariton) 7 and the all-normal-dispersion (dissipative soliton) regimes 8,9 . These are fundamentally different from the previously known soliton 10 and dispersion-managed soliton (stretched-pulse) 11 regimes. Here, we report a fibre laser in which the mode-locked pulse evolves as a similariton in the gain segment and transforms into a regular soliton in the rest of the cavity. To our knowledge, this is the first observation of similaritons in the presence of gain, that is, amplifier similaritons, within a laser cavity. The existence of solutions in a dissipative nonlinear cavity comprising a periodic combination of two distinct nonlinear waves is novel and likely to be applicable to various other nonlinear systems. For very large filter bandwidths, our laser approaches the working regime of dispersionmanaged soliton lasers; for very small anomalous-dispersion segment lengths it approaches dissipative soliton lasers.Passively mode-locked fibre lasers are being used in a diverse range of applications, including optical frequency metrology 12,13 , material processing 14 and terahertz generation 15 . Historically, major advances in laser performance have followed the discovery of new mode-locking regimes [1][2][3][4][5][6]16 , so there is always a strong motivation to search for new regimes.The physics of mode-locked fibre lasers comprises a complex interaction of gain, dispersion and nonlinear effects 17 . Such lasers are a convenient experimental platform for the study of nonlinear waves subject to periodic boundary conditions and dissipative effects. These characteristics profoundly alter the behaviour of nonlinear waves, so this area of research is interesting in its own right. In addition to the vast literature on optical solitons . These similaritons existed in segments of the cavity without any gain and loss to avoid the large spectral broadening that is characteristic of amplifier similaritons. Formation of a self-consistent solution in a laser cavity requires the compensation of spectral broadening, which has proved to be non-trivial 5 . Despite numerical predictions of their existence dating back almost a decade 26 , amplifier similaritons had yet to be observed in a laser cavity.Here, we present our experimental and theoretical work demonstrating an entirely new mode-locking regime, in which the pulse propagates self-similarly in the gain fibre with normal dispersion, and following spectral filtering, gradually evolves into a soliton in the rest of the cavity, where the dispersion is anomalous. All mode-locked lasers to date have had a single type of nonlinear wave propagating within the cavity; however, in our laser, distinctly different similariton and soliton pulses co-exist, demonstrating that transitions between these are possible. Remarkably, this construct is extremely robust against perturbations. Although the pulse expe...