  Bimetric gravity is a ghost-free and observationally viable extension of
general relativity, exhibiting both a massless and a massive graviton. The
observed abundances of light elements can be used to constrain the expansion
history of the Universe at the period of Big Bang nucleosynthesis. Applied to
bimetric gravity, we readily obtain constraints on the theory parameters which
are complementary to other observational probes. For example, the mixing angle
between the two gravitons must satisfy $\theta \lesssim 18^\circ$ in the
graviton mass range $m_\mathrm{FP} \gtrsim 10^{-16} \, \mathrm{eV}/c^2$,
representing a factor of two improvement compared with other cosmological
probes.
