Cauayan is a city in Isabela situated in the center of this big and naturally rich province.  The town got its name from the word “Cauayan” meaning bamboo in the dialect of Gaddang, the early settlers of this place.

Cauayan was an original town of the Province of Cagayan.  It was transferred to Nueva Vizcaya when it became a province in 1839.  Upon the creation of Isabela as a province by a Royal Decree issued on May 1, 1856, it was reverted as a town of the province.

Cauayan City is a fast-growing rural industrial center centrally located in the province and in the entire stretch out of Cagayan Valley.  It is about 375 kilometers northeast Manila; 34.5 kilometers south of Ilagan, the capital town of Isabela; about 48 kilometers north Santiago City; and roughly 117 kilometers away from Tuguegarao City in Cagayan Province.

San Guillermo is located in the southeastern part of the province of Isabela and is bounded on the north by the municipalities of Angadanan, Cauayan and San Mariano; on the south by the municipality of Echague, on the east by the municipality of Dinapigue and on the west by the municipality of Angadanan. 

San Isidro is a 5th class municipality that has 13 barangays.  A total of 7,200 hectares and approximately 65 kilometers distance from Ilagan, the Capital town of Isabela.