Career
Born in Sitio Bayog, Bgy.4, Bimmanga, Piddig, Ilocos Norte, he began swimming at the Guisit River as a small boy. He started competing in 1921 and collected a total of 144 medals over the next 16 years. During World World War II, Yldefonso fought against the Japanese in Bataan as part of the Philippine Scouts.
He survived the Bataan Death March, but later died at the Capas Concentration Camp.
His remains have never been recovered. He was named as part of the Hall of Fame by the International Swimming Federation in 2009.
Yldefonso"s unorthodox style in swimming which would be later known as the "Yldefonso Stroke" has been adapted by pint-sized swimmers, especially those from Asia, particularly the Japanese.