  The determination of the mean density of the Universe is a long standing
problem of modern cosmology. The number density evolution of x-ray clusters at
a fixed temperature is a powerful cosmological test, new in nature (Oukbir and
Blanchard, 1992), somewhat different from standard analyses based on the
dynamical measurement of individual objects. However, the absence of any
available sample of x-ray selected clusters with measured temperatures at high
redshift has prevented this test from being applied earlier. Recently,
temperature measurements of ten EMSS clusters at $0.3 \le z \le 0.4$ have
allowed the application of this test (Henry, 1997). In this work, we present
the first results of a new analysis we have performed of this data set as well
as a new estimation of the local temperature distribution function of clusters:
a likelihood analysis of the temperature distribution functions gives a
preferred value for the mean density of the universe which corresponds to 75%
of the critical density. An open model with a density smaller than 30% of the
critical density is rejected with a level of significance of 95%.
