  We present the first spatially resolved observations of a sample of 23 Mira
stars simultaneously measured in the near-infrared J, H and K' bands. The
technique used was optical long baseline interferometry, and we present for
each star visibility amplitude measurements as a function of wavelength. We
also present characteristic sizes at each spectral band, obtained by fitting
the measured visibilities to a simple uniform disk model. This approach reveals
the general relation J diameter < H diameter < K' diameter.
