In double-heterostructure stripe-geometry semiconductor lasers an effective lateral index step over the stripe region can be induced through evanescent-field coupling. Such a quasi-index-guided device exhibits a transition from the gain-guided to the index-guided regime when is progressively increased. Using parameters appropriate to a 1.3-μm InGaAsP laser, the transition is shown to occur around . The exact value of depends on the extent of carrier-induced antiguiding. In the transition region the threshold current decreases rapidly, the lateral mode contracts, and the far field changes from a twin-lobe to a single-lobe pattern. Our analysis suggests that a quasi-index-guided device operates most efficiently for values of at which the index-guided regime is just approached. With a further increase of , the mismatch between the gain and mode profiles leads to lower differential quantum efficiencies. Among other structures, the analysis is applicable to a ridge waveguide laser. For a 1.3-μm laser the optimum can be obtained using 0.2-μm-thick cladding layers for a 0.2-μm thick active layer.