A natural approach to construct a prime theta-curve is to an add an arc to a prime knot. We study that approach for knots and arcs on a standard torus in the 3-sphere. We prove that each nontrivial torus knot union an essential arc is a prime theta-curve. By comparing the constituent knots of those theta-curves, we obtain infinitely many prime theta-curves. We then classify all theta-curves that lie on a standard torus. In particular, Kinoshita's theta-curve does not lie on a standard torus.
