We report on a study of the morphology of (100) silicon surfaces etched in aqueous alkaline solutions. It is shown that the formation of pyramidal hillocks during etching can be influenced in two different ways: by the presence of an oxidizing agent (ferricyanide or oxygen) in the etchant solution, or by etching under anodic bias. In both cases pyramid formation is suppressed without a significant change of the etch rate. The addition of ferricyanide does not markedly change the etching anisotropy. The formation, stability, and suppression of the pyramids are discussed.