Pacinotti spent the war defending Italian shipping around Italy.

The Regia Marina (the Royal Italian Navy) placed an order for the two submarines in 1913 from the Società FIAT-San Giorgio, Muggiano, La Spezia while SM U-42 (later renamed Balilla) was being built.

][3][4] The Gugliemotti, while on its maiden voyage, was mistaken for a German U-boat by HMS Cyclamen northwest of Capraia on 10 March 1917 where it was sunk by both gunfire and ramming.

The Pacinotti operated in the Tyrrhenian Sea, defending trade routes, for a total of 13 missions between 24 March 1917 and 4 September 1917.

It was then deployed to Sicily on 7 September 1917, undertaking anti-submarine patrols along with the 300 GRT Malamocco, which acted as bait for enemy submarines.