This paper presents a detailed study of the cw propeties of a modified‐strip buried‐heterostructure (MSBH) laser. Included are studies of the electrical, electro‐optical, and optical beam properties, together with spectral characteristics, and temperature dependence of thresholds. A variation of this type of laser, a passive‐mirror modified‐strip buried heterostructure is also described. The MSBH laser possesses many desirable properties including excellent linearity of the lasing emission consistent with a second‐harmonic content more than 40 dB below the fundamental, constant proportionality between the light emitted from the two mirror facets, ’’kink‐free’’ emission to the maximum cw power levels measured (35 mW/mirror facet), nearly round near‐field intensity distribution with an ellipicity of 1.3 : 1, lasing emission in the lowest‐order transverse mode, full angle half‐power perpendicular to the junction plane of 27.3° (significantly smaller than any other practical cw DH laser structure), and narrow cw spectra (emission is typically in only a few longitudinal modes). MSBH lasers are nonastigmatic signal sources so that diffraction‐limited spherical optics can be used. Pulsations are not observed if the active stripe is free of absorbing defects. cw operation of 10 mW/mirror facet was obtained at ambient temperatures to 90 °C with aT0of 126 °K. For 380‐&mgr;m‐long lasers pulsed threshold currents of 78 mA dc, slope efficiencies of 63%, forward resistances of 2 &OHgr;, and zero‐biased capacitances of 15 pF have been measured. At the operating power of 3 mW/mirror facet, the optical power density at the mirror facet is low (?104W/cm2) which may have positive reliability implications. The passive‐mirror form of the MSBH laser (PM‐MSBH laser), with the potential for avoiding surface effects which have been implicated as a cause of pulsations and laser degradation, have been measured with thresholds of 104 mA dc and slope efficiencies of 31%.