Excessive amounts of starch in diets for lactating dairy cattle is a known risk factor for milk fat depression but little is known about how these risks are altered by differences in rates of starch degradability (K_d) in the rumen. The objective of this study was to compare accumulation of biohydrogenation intermediates causing milk fat depression, including conjugated linoleic acid (CLA), when corn with low or high K_d were fed to continuous cultures. Diets contained (DM basis) 50% forage (alfalfa pellets and grass hay) and 50% concentrate with either no added fat (LF) or 3.3% added soybean oil (HF). Three LF and 3 HF diets contained corn sources with either low, medium, or high K_d (48.4, 66.2, or 84.0% in a 7 h in vitro test) giving 6 diet treatments with a 2 × 3 factorial arrangement. Each diet was fed to dual-flow continuous fermenters 3 times daily at 0800, 1600, and 2400 h. Diets were fed for 4 10 d periods, with 7d for adaptation and 3 d for sample collection. No fat × starch interactions occurred (P > 0.05) so main effects are presented. Starch effects were tested by linear (L) or quadratic (Q) contrasts. The LF and HF treatments differed (P < 0.05) in acetate (53.0 vs. 49.0 mol/100 mol), acetate/propionate (2.00 vs. 1.70), trans-10 18:1 (2.7 and 13.6% of total), and trans-10,cis-12 CLA (0.15 and 0.41% of total), respectively. Increasing starch K_d from low to high increased (P < 0.05) culture pH (L, 6.22, 6.23, 6.38) and acetate (Q, 48.7, 50.6, 53.9 mol/100 mol) but decreased (P < 0.05) butyrate (Q, 16.4, 15.0, 12.2) mol/100 mol). Changes in biohydrogenation intermediates (expressed as % of total fatty acids) from low to high K_d included decreases (P < 0.05) in trans-11 18:1 (Q, 8.58, 4.78, 4.46) and cis-9,trans-11 CLA (Q, 0.40, 0.34, 0.20) but an increase (P = 0.07) in trans-10,cis-12 CLA (L, 0.23, 0.24, 0.37). The results show that increasing the starch K_d in continuous culture while holding starch level constant causes elevation of some biohydrogenation intermediates linked previously to milk fat depression.