Tabulated values of F-layer critical frequency (foF2), that were scaled from ISIS-2 topside ionograms, have been used to study the main F-layer trough. These foF2 values were selected because they contained good examples of the trough near midnight, in mid-winter, and under quiet and disturbed geomagnetic conditions. The study shows that the steep electron density gradient that characterizes the poleward wall of the trough is typically equal to 43,000 electrons cu cm per deg. The wall extends over about 3 deg of latitude. At the latitude of the base of the poleward trough wall, the electron density is typically 48,000 electrons cu cm (foF2 = 2.0 MHz). The trough is 4 deg wide in lattitude at 2 MHz; that is, foF2 values remain at or below MHz for about 4 deg of lattitude equatorward from the poleward trough wall. Likewise, the trough is 11 deg wide at 2.5 MHz and 19 deg wide at 3.0 MHz. An equation is given in the test that can be used to characterize foF2 values in the trough in near real-time based on readily observable geophysical parameters. (Author).