A polymer microsphere for use as a fluid loss additive in drilling muds including a hydrophobic core of hydrophobic monomers and a hydrophilic shell of hydrophilic monomers wherein the hydrophilic shell surrounds the hydrophobic core. The polymer microsphere is capable of forming micelles or microgels in water without using surfactants and can be block, graft, and random copolymers. The hydrophilic shell is physically or chemically linked to the hydrophobic core. The polymer microsphere further includes crosslinkers, preferably where the crosslinkers are monomers containing at least two ethylenically unsaturated groups.
