Ducting by magnetic-field-aligned density irregularities is found to occur in seven regions of parameter space defined by plasma resonance (P=0), unity refractive index (no=1), longitudinal refractive-index curvature reversal (no2=-P),=-P), and the boundary between ordinary and extraordinary propagation (no2=+P), where no2=R or L=R is the squared refractive index in the direction of the magnetic field and P, R, and L are the Stix parameters for wave propagation in a plasma. Ducting in the ordinary and extraordinary modes is found to occur in troughs, whistler ducting is found to occur in troughs and crests respectively above and below its curvature reversal at approximately half the cyclotron frequency, and ducting in the Z mode is found to occur in troughs from the Z cutoff to a curvature reversal about halfway between cutoff and the plasma frequency, in crests up to the plasma frequency, and then also in crests between the plasma and cyclotron frequencies whenever the latter is greater than the former. The strength of ducting, which is characterized by the square of the ducted wave angle divided by the fractional density deviation of the duct, is found to be proportional to the refractive-index radius of curvature and the rate of increase of refractive index with density. Strongest ducting is thus found to occur in the Z mode below curvature reversal, and the weakest at low densities near the plasma frequency. ¿ American Geophysical Union 1995