Of the more band-efficient constant envelope digital modulations, duobinary FM is attractive in terms of its potential error performance, as indicated by the minimum distance of the signal structure. In this paper, the error performance of narrow-band duobinary FM, with discriminator detection in an additive Gaussian noise background, is evaluated for a range of system bandwidthsBTand frequency deviation ratiosh. The effects of intersymbol interference, due to the restricted bandwidth, are taken into account in the analysis. In addition, at the narrower bandwidths,0.5 leq BT < 1.0, the filtered background noise is correlated, and these effects are included. It is shown that with the usual duobinary detector, "clicks," which are a major contributor to errors in conventional digital FM, do not contribute significantly to errors for the low frequency deviation ratios which are commonly used. The error rate theory agrees well with results obtained experimentally. The deviation ratios that give the best error performance for a given system bandwidth are presented. In a narrower bandwidth such asBT = 1/2, where duobinary FM is proposed, duobinary FM with a frequency deviation ratio ofh = 0.7performs better than binary FM.