Abstract We point out that dynamics known from the observed CP violation in K L → π ∓ l ± ν coupled with CPT invariance generate a CP asymmetry of 3.3 × 10 −3 in τ ± → ν K S π ± → ν [ π + π − ] K π ± . The equality of the τ + and τ − lifetimes required by CPT symmetry is restored in an intriguing way as the combined effect of long-lived K → π + π − as well as contributions from K L – K S interference. While little new can be learnt from this CP asymmetry, the latter has to be accounted for, since CP asymmetries in τ → ν K π channels are prime candidates for revealing the intervention of New Physics. This ‘known’ CP asymmetry provides a very useful calibration point in such searches. It also provides a test of CPT symmetry (as well as the Δ Q = Δ S selection rule).