We investigate the magnetostriction of field-structured magnetoelastomers, which are an important class of materials that have great potential as both sensors and actuators. Field-structured magnetoelastomers are synthesized by suspending magnetic particles in a polymeric resin and subjecting these to magnetic structuring fields during polymerization. These structuring fields can consist of as many as three orthogonal ac components, allowing a wide variety of particles structures--chains, sheets, or networks--to be formed. A principal issue is how particle structure and loading affects the magnetostriction of these materials. To investigate magnetostriction in these field-structured composites we have constructed a constant stress, optical cantilever apparatus capable of 1 ppm strain resolution. Magnetoelastomers having a wide range of particle loadings and structures are investigated, and it is shown that the observed deformation depends strongly on composite structure. The best magnetoelastomers exhibit a contractive strain of 10,000 ppm, the worst materials exhibit a negative, tensile response, which we show is due to the dominance of demagnetizing field effects over magnetostriction. Finally, some discussion is given to the surprising finding that magnetostriction is proportional to the sample prestrain. Simulations of a chain of particles in an elastomer show that particle clumping transitions can occur, but this does not account for the dependence of magnetostriction on prestrain.