The phase behavior of tethered polymer monolayers is studied theoretically. These layers are formed at the air-water interface by hydrophobic polymers with a hydrophilic endgroup, the effect of the endgroup being to anchor the end of the polymer at the interface, forming a brush. A mixed layer of two incompatible polymers should exhibit modulated phases, arising from the competition between demixing and electrostatic forces. The phase behavior is calculated in terms of the density of polymer and the degree of polymerization. Stripe phases are found to occur for sufficiently high molecular weight.