We have developed a one-dimensional photochemical model to study both the composition and density structure of Titan's ionosphere. The model neutral atmosphere developed by Yung et al. (1984) and Yung (1987) was used as a basis to model the chemistry of Titan's ionosphere. Ionization rates due both to photoionization by solar EUV flux and to electron impact ionization by photoelectrons and Saturnian magnetospheric electrons were included. The major neutral species (nitrogen and methane) are ionized to produce N2+, N+, CH4+, CH3+, CH2+, and CH+ ions. Ion-neutral chemistry converts these ions to the major ion species H2CN+ (in agreement with Atreya (1986), Strobel (1985), and Ip (1990a)), as well as C2H5+, CH5+, HCN+, and more complex hydrocarbon ions (Cn Hm+). Inclusion of both forms of ionization gave a peak electron density of ≈3030 cm-3 at an altitude of ≈1175 km along the terminator. The radio occultation experiment on board Voyager 1 (Lindal et al., 1983) put limits on the maximum ionospheric electron density of 5¿103 cm-3 and 3¿103 cm-3 along the morning and evening terminators, respectively. The total external pressure (i.e., dynamic, thermal, and magnetic) upstream of Titan at the time of the Voyager encounter is of the order of the maximum ionospheric thermal pressure. As a consequence one can expect that the solar wind interaction with Venus during periods of high solar wind dynamic pressure might provide a good analogy for the interaction of Titan with the Saturnian magnetospheric plasma. ¿ American Geophysical Union 1992