Longitudinal-mode behaviour of λ/4-shifted InGaAsP/InP DFB lasers fabricated by the nega-posi method have been studied. The deviation of the main mode from the centre of the stopband, which was attributed to an accidental additional phase shift introduced around the λ/4-shifted position, was related to the intensity ratio of the main to side mode below the threshold, and it was much larger compared to conventional DFB lasers for the devices with small deviation. The intensity ratio above the threshold was about 35 dB or more in 100% direct modulation for most of the lasers.