A coriolis-type mass flow sensor (10) suitable for aerospace use comprises a turning fork (14) which is immersed in the fluid whose mass flow is to be sensed, such that its vanes extend across (ie perpendicular to) the direction of flow of the fluid. The whole tuning fork is elongated in the direction of flow of the fluid, so that the fluid flows over (and between) the vanes: typically, the fork may be up to 15 cm long in the direction of flow. The vanes are excited to vibrate perpendicularly to the direction of flow of the fluid by several piezoelectric drive devices held under compression in, and distributed symmetrically about the midlength of, the yoke (12) from which the vanes project, while the vibrations are sensed by respective piezoelectric pickup devices within each end of the yoke. The phase difference between the vibrations sensed by the two pickup devices is a function of the mass flow of the fluid. In an alternative form, the sensor has two pairs of vanes symmetrically distributed around the yoke as a kind of double tuning fork, while in another alternative form, the vanes are formed integrally with, and project into, a short section of cylindrical pipe.