  Violations of CPT invariance can induce neutrino-to-antineutrino transitions.
We study this effect for solar neutrinos and use the upper bound on the solar
neutrino-to-antineutrino transition probability from the KamLAND experiment to
constrain CPT-symmetry-violating coefficients of the general Standard-Model
Extension. The long propagation distance from the Sun to the Earth allows us to
improve existing limits by factors ranging from about a thousand to $10^{11}$.
