  Following the announcement of the detection of phosphine (PH$_3$) in the
cloud deck of Venus at millimeter wavelengths, we have searched for other
possible signatures of this molecule in the infrared range.
  Since 2012, we have been observing Venus in the thermal infrared at various
wavelengths to monitor the behavior of SO$_2$ and H$_2$O at the cloud top. We
have identified a spectral interval recorded in March 2015 around 950 cm$^{-1}$
where a PH$_3$ transition is present.
  From the absence of any feature at this frequency, we derive, on the
disk-integrated spectrum, a 3-$\sigma$ upper limit of 5 ppbv for the PH$_3$
mixing ratio, assumed to be constant throughout the atmosphere. This limit is 4
times lower than the disk-integrated mixing ratio derived at millimeter
wavelengths.
  Our result brings a strong constraint on the maximum PH$_3$ abundance at the
cloud top and in the lower mesosphere of Venus.
