A new experiment has been set up at the Paul Scherrer Institut to sear
ch for muonium to antimuonium conversion. No event was found to fulfil
the requested signature which consists of the coincident detection of
both constituents of the antiatom in its decay. Assuming an effective
(V - A) x (V - A) type interaction an improved upper limit is establi
shed for the conversion probability of P(<M(M)over bar>) less than or
equal to 8 x 10(-9) (90% C.L.), which is almost 2 orders of magnitude
lower compared to previous results and provides a sensitive test for t
heoretical extensions of the standard model.