In a recent multicenter, random-order, home-use trial (NCT03565666), we examined automated glycemic control with the insulin-only configuration of the bionic pancreas (BP) using subjects’ typical insulin analogue, BP-T (aspart, n=7 or lispro, n=27), versus using the ultra-rapid faster aspart, BP-F. The dosing algorithm was identical in both arms and used the same setting for insulin absorption speed (tmax=65 minutes). The mean CGM glucose (CGMG) was similar between the BP-F and BP-T arms (154±11 vs. 155±13 mg/dl), as was the median percent time <54 mg/(0.6% [0.2-1.1%] vs. 0.6% [0-1.3%]). We used an autoregressive time series model to determine statistical significance for differences between the BP-F and BP-T arms in individual subjects. Nine subjects had a significantly lower mean CGMG during the BP-F arm (by 13±8 mg/dl), 10 subjects had a significantly higher mean CGMG during the BP-F arm (by 10±5 mg/dl), and 15 subjects had no significant difference between arms (nominal difference 2±2 mg/dl). One subject with a significantly lower mean CGMG in the BP-F arm also had a significant decrease of 1.4% in time <54 mg/dl (~20 minutes per day), one subject with no difference in mean CGMG between arms had a significant increase of 1.4% in time <54 mg/dl in the BP-F arm, and 32 subjects had no significant differences in time <54 mg/dl between arms. There was a statistically significant reduction in mean CGMG from the use of Fiasp without adjustment of the setting for insulin absorption speed for some subjects, but a similar number saw a significant increase in mean CGMG, while there was no difference for the remainder of subjects. Other studies are under way to test whether adjusting the BP insulin delivery algorithm tmax setting will allow more subjects to benefit from the use of faster aspart in the iLet. Disclosure J. Sherwood: None. H. Zheng: None. C.A. Balliro: None. L. Ekhlaspour: None. B.A. Buckingham: Advisory Panel; Self; ConvaTec Inc., Medtronic. Research Support; Self; Beta Bionics, Inc., Dexcom, Inc., Insulet Corporation, Medtronic, Tandem Diabetes Care. F. El-Khatib: Other Relationship; Self; Beta Bionics, Inc. E. Damiano: Board Member; Self; Beta Bionics, Inc. Employee; Self; Beta Bionics, Inc. Stock/Shareholder; Self; Beta Bionics, Inc. Stock/Shareholder; Spouse/Partner; Beta Bionics, Inc. S.J. Russell: Consultant; Self; Beta Bionics, Inc., ConvaTec Inc., Novo Nordisk A/S, Senseonics. Research Support; Self; Beta Bionics, Inc., Novo Nordisk A/S, Zealand Pharma A/S. Stock/Shareholder; Self; Companion Medical. Other Relationship; Self; Ascensia Diabetes Care, Roche Diabetes Care. Funding National Institutes of Health