Lifetimes in ¹⁸⁸Pb were measured using the Köln plunger in combination with the RITU separator and JUROGAM. Four lifetimes were measured, from which the deformation of the prolate band was experimentally determined for the first time. The squared prolate mixing amplitude of the 2₁⁺ state was deduced from the measured B(E2) values. ; This work was supported in part by the U.S.D.O.E. under grant No. DE-FG02-91ER-40609, the Academy of Finland (the Finnish Centre of Excellence Programme, project 44875), the EU-FP5 projects EXOTAG (HPRI-1999-CT-50017) and Access to Research Infrastructure (HPRI-CT-1999-00044), and the BMBF (Germany) under Contract No. 06K167.