A new, generic method to exercise control over the shape of crystallites is reported. Crystals of PbS are grown in the unusual form of pyramids at the water-toluene interface. The pyramids are single crystalline and adopt a unique growth habit (slow growth along [113] direction). The pyramids are exclusive products of the reaction and are obtained in the form of a monolayered film spread across the fluid interface. The origins of the growth habit and assembly lie in phenomena unique to the liquid-liquid interface. The dimensions of the pyramidal base can be controllably varied in the range 575-1260 nm. Crystallites of other forms such as rods and spheres can be obtained by varying the properties of the fluid interface.