An InGaAsP 1.5 μm laser, with one facet antireflection coated, has been incorporated into a diffraction grating external cavity. The lasing wavelength could be tuned over a 55 nm range about the centre wavelength of 1.5 μm by rotating the grating. Furthermore, it was found that the emission spectrum was extremely narrow; beat-frequency measurements at 1523 nm against an HeNe laser showed it to be of the order of 10 kHz.