  The existence of a light or massive scalar field with a coupling to matter
weaker than gravitational strength is a possible source of violation of the
weak equivalence principle. We use the first results on the E\"otv\"os
parameter by the MICROSCOPE experiment to set new constraints on such scalar
fields. For a massive scalar field of mass smaller than $10^{-12}$ eV (i.e.
range larger than a few $10^5$ m) we improve existing constraints by one order
of magnitude to $|\alpha|<10^{-11}$ if the scalar field couples to the baryon
number and to $|\alpha|<10^{-12}$ if the scalar field couples to the difference
between the baryon and the lepton numbers. We also consider a model describing
the coupling of a generic dilaton to the standard matter fields with five
parameters, for a light field: we find that for masses smaller than
$10^{-12}$eV, the constraints on the dilaton coupling parameters are improved
by one order of magnitude compared to previous equivalence principle tests.
