Highly dispersive photonic band-gap-edge optofluidic biosensors are studied theoretically. We demonstrate that these structures are strongly sensitive to the refractive index of the liquid, which is used to tune dispersion of the photonic crystal. The upper frequency band-gap edge shifts about 1.8~nm for $\delta n=0.002$, which is quite sensitive. Results from transmission spectra agree well with those obtained from the band structure theory. © The Authors. All rights reserved [DOI: 10.2971/jeos.2006.06026]