September 1937 the Nyon Conference was called by France and Great Britain to address the "underwater piracy" conducted against merchant traffic in the Mediterranean. On September 14, an agreement was signed establishing British and French patrol zones around Spain (with a total of 60 destroyers and airforce employed) to counteract aggressive behavior by submarines. Italy was not directly accused, but had to comply with the agreement and suspend the underwater operations. Under pressure from Franco's regime, Italy decided to transfer 4 more submarines (in addition to and already being operated by the Falangists) to the Spanish Legion (Legión Española or Tercio de Extranjeros). Onice was one of the four boats chosen for the transfer. On September 17, 1937 Onice left La Maddalena and headed to Soller on Mallorca, and arrived there on September 23, 1937. She was placed under the direct command of Spanish admiral Francisco Moreno, was renamed Aguilar Tablada and assigned pennant number L4. However, Onice retained her commander (captain Mario Ricci), senior officers and Italian crew, but they had to wear Spanish uniforms and insignia. The other three Italian submarines transferred to Tercio were (Gonzalez Lopez), (General Mola II) and (General Sanjurjo II). All four were based at Soller. Onice (Aguilar Tablada) became the first to start operations, serving in "Tercio" for four months, and conducting 3 missions. During October 8 through 18, 1937 Onice carried out her first mission under the Nationalist flag starting between Benidorm and Alicante (until October 12) and then off Cartagena. There were several occasions to attack, but they all were wasted for various reasons, along with wasted torpedoes which resulted in heavy criticism of Captain Ricci and his apparent lack of expertise. During November 2 through 11, 1937 Onice carried out her second mission patrolling off Tarragona under a new commander, captain Alfredo Criscuolo. She attacked one steamer, but a failure of the echo sounder and the hydrophones forced Onice to return to Cagliari for repairs. During January 31 through February 4, 1938 she carried out her third and last mission, under command of captain Manlio Petroni, off Tarragona but without any results. On February 5, 1938, she left Soller and returned to Italy and assumed her old name of Onice ending her "legionary" career. In 1939 she was based at La Spezia, and later at the Red Sea base of Massawa in Eritrea together with Iride and . She returned