  We present our discovery of a narrow-line Baldwin effect, an anti-correlation
between the equivalent width (EW) of a line and the flux of the associated
continuum, in 5-20$\mu$m mid-infared lines from a sample of 68 Active Galactic
Nuclei (AGN), located at z$<$0.5, observed with the Infrared Spectrograph on
the {\it Spitzer Space Telescope}. Our analysis reveals a clear
anti-correlation between the EW of the [SIV] 10.51$\mu$m, [NeII] 12.81$\mu$m,
and [NeIII] 15.56$\mu$m lines and their mid-IR continuum luminosities, while
the Baldwin effect for [NeV] 14.32$\mu$m is not as obvious. We suggest that
this anti-correlation is driven by the central AGN and not circumnuclear star
formation in the host galaxy. We also find that the slope of the narrow-line
Baldwin effect in the mid-infrared does not appear to steepen with increasing
ionization potential. Examining the dependence of the EW to the Eddington Ratio
($L/L_{Edd}$) we find no strong relationship for mid-IR lines. Our study
indicates that the narrow-line mid-infrared Baldwin Effect is quite different
from the broad-line optical/UV Baldwin effect and it is possible that the two
effects are unrelated. The discovered anti-correlations open new possibilities
in the understanding the physics of the ionizing region and the continuum
reprocessing by dust.
