Westiellopsis prolifica Janet and Chaetophora attenuata Hazen cultures released sugars (glucose, fructose, and sucrose), organic acids (oxaloacetic acid and oxalic acid), amino acids, and protein. W. prolifica cultures released the amino acids glycine, serine, cystine, glutamic acid, aspartic acid, and α-alanine, while C. attenuata cultures released glycine, serine, aspartic acid, and α-alanine. W. prolifica and C. attenuata cultures of all ages released more extracellular protein than total free amino acids. Cultures of C. attenuata released more protein than cultures of the same age of W. prolifica. The filtrates from old cultures of W. prolifica and C. attenuata decreased the total chlorophyll content of all algae tested, totally suppressed conjugation in Spirogyra decimino and zoospore formation in C. attenuata, and drastically decreased spore germination in W. prolifica, thus producing stressful conditions affecting the growth and reproduction of these and other algae.