Nematogens rotate by the application of external fields, thereby enabling optical modulation. This principle has had a profound impact on our daily lives through the plethora of liquid-crystal displays in use around us(1,2). However, the wider use of nematic liquid crystals, particularly in microdisplays(3) and information processing, has been hampered by their slow response times. In nematogens, rotational and translational molecular motions are coupled(4), so flow is inevitably linked with optical modulation(5,6). This linkage motivated us to fuse microfluidics with anisotropic liquids and introduce an optofluidic(7,8) modulator that exhibits a submillisecond (250 mu s) symmetric response and can operate at frequencies up to 1 kHz. The modulator is based on peristaltic nematogen microflows(9) realized in polydimethylsiloxane microfluidics. The latter simultaneously permits peristalsis by means of elastomeric deformation, nematogen alignment and rapid prototyping through cast-moulding. Together with large-scale, vertical integration and piezoelectric nanotechnologies, this optofluidic paradigm can enable high-density and three-dimensional architectures of fast modulators.