Misfit dislocations in epitaxially grown layers of GaAs1−xPx with a lattice constant gradient are examined by transmission electron microscopy. In specimens with (113) A growth planes, they form a three-dimensional arrangement of glissile and sessile dislocations. Cross slip is an important process in the generation of the dislocations. High resolution microscopy shows 1) glissile dislocations dissociated into partial dislocations and 2) undissociated sessile Lomer dislocations. These differences are attributed to contributions to the dislocation core energy from wrong bonds and dangling bonds. Screw dislocations are also thought to be undissociated, which facilitates cross slip and multiplication of dislocations.