One of the least controlled defects in Swiss cheese is development of splits. Split defect is characterized by fissures or cracks in the body of the cheese that can be as short as 1 cm in length or long enough to span a 90-kg block. This defect appears during refrigerated storage after the cheese is removed from the warm room. Swiss cheese with splits is downgraded because it is unsuitable for use on high-speed slicing equipment (up to 1,000 slices per minute). A 2x2x2 factorial experiment was used to determine the effect of different commercial Lactobacillus helveticus starters combined with commercial gas-forming strains of Propionibacterium freudenreichii ssp. shermanii on the occurrence of split defect in Swiss cheese. Two strains of L. helveticus recommended for Swiss cheese manufacture were used along with two strains of P. freudenreichii ssp. shermanii. The same strain of Streptococcus thermophilus was used in all treatments. To investigate the influence of seasonal variations in milk supply, eight vats were made in the summer and eight vats were made in the winter, each producing five 90-kg blocks of cheese. Each 90-kg block of cheese was cut into twenty-four 4-kg blocks, and each 4-kg block was graded based on the presence of splits. If splits were present, the cheese was downgraded from A to C grade. Only small variations were found in the composition of cheeses made during the same season. There were no correlations between cheese moisture, pH, fat, protein, calcium, lactose contents, D/L lactate ratio, or protein degradation that could be used to predict the amount of splits present after 90 d of storage. The extent of split formation was influenced by both the L. helveticus and P. freudenreichii ssp. shermanii cultures used. In this study, we were able to show a fivefold reduction in downgraded cheese through proper culture selection from 90% to 14% in the summer cheese. Even though less than 6% of the cheese split in the winter, the culture effect was nonetheless repeatable with a similar reduction through culture selection from 6% to 1% in winter cheese. Split formation also increases with storage time. If a cheese has a tendency to split, there will be a higher percentage of downgraded cheese the longer it is kept in storage.