Single heterojunction photodiodes suitable for the 1.0 to 1.3 pm spectral region of interest for fiber optics communication have been prepared from lattice-matched p-type epilayers of the quaternary III-V alloy semiconductor InGaAsP on n-type InP substrates. The characteristics of these heterojunctions are demonstrated by mesa photodiodes made from Zn-doped epilayers of composition In0.84Ga0.16Aso.34P0.66 on Sn-doped InP. Their room temperature spectral response extends from the ~0.96 um self-filtering cutoff of the InP substrate to (~1.13 um determined by the bandgap of this particular quaternary composition. Responsivities of 0.46 A/W and external quantum efficiencies of 0.54 are measured at 1.05 um.