A simple method has been devised to determine dynamic changes in volume of biological objects of a ‘smooth’ form. The method requires two perpendicular views (side and bottom). The method can be used also in static volume changes. Differential changes in length, width and height can be quantified. The volume of the object is approximated by a series of ellipses. The length of the axes of the ellipses is determined from the two views. Possible sources of error are discussed. The method is applied in a study of static volume changes of fish larvae due to fixation and in a study of the dynamic volume change of the mouth cavity of carp larvae, increasing their head volume 30% in 8 ms, when sucking prey.