  We study longwave Marangoni convection in a layer heated from below. Using
the scaling $k = O(\sqrt{\rm Bi})$, where $k$ is the wavenumber and ${\rm Bi}$
is the Biot number, we derive a set of amplitude equations. Analysis of this
set shows presence of monotonic and oscillatory modes of instability.
Oscillatory mode has not been previously found for such direction of heating.
Studies of weakly nonlinear dynamics demonstrate that stable steady and
oscillatory patterns can be found near the stability threshold.
