For any strictly convex planar domain Ω ⊂ R 2 with a C ∞ boundary one can associate an infinite sequence of spectral invariants introduced by Marvizi-Merlose [5]. These invariants can generically be determined using the spectrum of the Dirichlet problem of the Laplace operator. A natural question asks if this collection is sufficient to determine Ω up to isometry. In this paper we give a counterexample, namely, we present two non-isometric domains Ω andΩ with the same collection of Marvizi-Melrose invariants. Moreover, each domain has countably many periodic orbits {S n } n 1 (resp. {S n } n 1 ) of period going to infinity such that S n andS n have the same period and perimeter for each n.Consider a C ∞ smooth strictly convex planar domain Ω ⊂ R 2 . Let us start by introducing the Length Spectrum of a domain Ω. The length spectrum of Ω is given by the set of lengths of its periodic orbits, counted with multiplicity:L(Ω) := N{ lengths of closed geodesics in Ω} ∪ N |∂Ω|, where |∂Ω| denotes the length of the boundary of Ω. Generically this collection can be determined from the spectrum of the Laplace operator in Ω with Dirichlet boundary condition (similarly for Neumann boundary one):(1) ∆f = λf in Ω f | ∂Ω = 0.From the physical point of view, the eigenvalues λ's are the eigenfrequencies of the membrane Ω with a fixed boundary. There is the following relation between the Laplace spectrum and the length spectrum (see e.g. [1,6]). Call the functionthe wave trace. Then, the wave trace w(t) is a well-defined generalized function (distribution) of t, smooth away from the length spectrum, namely,(2) sing. supp. w(t) ⊆ ±L(Ω) ∪ {0}.So if l > 0 belongs to the singular support of this distribution, then there exists either a closed billiard trajectory of length l, or a closed geodesic of length l in the boundary of the billiard table.