Measurements of electron density at seven D region altitudes were made with the Arecibo radar during a class X solar flare on August 18, 1979. Measurements of solar X ray fluxes during the same period were available from the GOES 2 satellite (0.5--4 ¿ and 1--8 ¿) and from ISEE 3 (in four bands between 26 and 400 keV). From the X ray flux data we computed ionization rates in the D region and the associated chemical changes, using a coupled atmospheric chemistry and diffusion model (with 997 chemical reactions and 19 vertical levels). The computed electron densities matched the data quite well after we had adjusted the rate coefficients of three reactions, namely, O2-+O2(1Δ)→e+O2+O2, O2++O2+M→O4++M, and O4++O→O2++O3. We also discuss the hierarchies among the many flare-induced chemical reactions in two altitude ranges within the D region and the possible effects of two separate isomeric forms of O4+. ¿ American Geophysical Union 1990