In late January 1971 he was bought by Basque giants Athletic Bilbao, making his official debut on the 31st in a 2–0 La Liga home win over RC Celta de Vigo, with his team eventually finishing fifth and qualifying for the UEFA Cup.

During his spell with the club he started as a full back, moved to central defender, was also deployed as a defensive midfielder and finished again in the defensive wings; in two consecutive seasons under Milorad Pavić he scored ten league goals combined, with Bilbao also winning the 1973 edition of the Copa del Rey.

[2] During the better part of his career, Guisasola struggled with weight problems, and his production was highly inconsistent in his final years.

An injury to starter José Ramón Alexanko – later of FC Barcelona fame – propelled him to the starting XI once again, and he helped Athletic finish seventh in the 1979–80 season; as they won the league in 1983, under Javier Clemente, he only contributed with four appearances (151 minutes).

On 26 March 1980, Guisasola played his first and only game with the Spain national team, starting in a 0–2 friendly loss to England at the Camp Nou.