EXHIBIT 21.1 Subsidiaries of the Registrant Patterson (GP) LLC Patterson (GP2) LLC Patterson (LP) LLC Patterson-UTI Drilling Company LP, LLLP Patterson-UTI Drilling Company South LP, LLLP Patterson-UTI Drilling Company West LP, LLLP Patterson Petroleum LP, LLLP Lone Star Mud LP, LLLP Ambar Drilling Fluids LP, LLLP Patterson Petroleum Trading Company LP, LLLP Analytical Seismic Systems, Inc. UTICO, Inc. SUITS Drilling Company UTI Management Services, L.P. UTI Drilling, L.P. UTICO Hard Rock Boring, Inc. International Petroleum Service Company Universal Well Services, Inc. Norton Drilling Services, Inc. Norton Drilling Mexico Norton Drilling, L.P. Norton GP, L.L.C. Phelps Drilling Co. UTI Drilling Canada, Inc.