Derivator is a PDP-1 program for examining the solutions to differential equations by inspection of a visual display of trajectories. Because fixed-point arithmetic is used (in order to maintain visual display speeds), Derivator must be regarded as a qualitative tool. It is subject to truncation error in the trajectory-following program, and round-off error due to 'underflow' in the function-definition programs for dy and dx. Still it appears to be very suitable for studying topology of solutions around singularities, etc. The display shows the solution curves ('characteristics') in the x-y plane. They are generated parametrically.