Gene Garber (4–3) || Warren Brusstar (0–1) || None || 6,232 || 8–14(42–35) |- style="background:#fbb" | 78 || September 3 || Reds || 3–9 || Bruce Berenyi (7–4) || Mark Davis (0–3) || None || 26,540 || 8–15(42–36) |- style="background:#bfb" | 79 || September 4 || Reds || 7–6 || Sparky Lyle (7–4) || Doug Bair (2–2) || Tug McGraw (9) || 25,020 || 9–15(43–36) |- style="background:#bfb" | 80 || September 5 || Reds || 5–4 || Ron Reed (3–1) || Mario Soto (8–8) || None || 41,845 || 10–15(44–36) |- style="background:#fbb" | 81 || September 6 || Reds || 4–5 || Tom Hume (7–3) || Sparky Lyle (7–5) || Joe Price (2) || 30,366 || 10–16(44–37) |- style="background:#fbb" | 82 || September 7 || Expos || 4–5 || Woodie Fryman (5–2) || Mike Proly (2–1) || Jeff Reardon (5) || 31,401 || 10–17(44–38) |- style="background:#bfb" | 83 || September 8 || Expos || 10–5 || Dan Larson (1–0) || Scott Sanderson (7–6) || None || 11,812 || 11–17(45–38) |- style="background:#bfb" | 84 || September 9 || Expos || 11–8 || Ron Reed (4–1) || Woodie Fryman (5–3) || Sparky Lyle (1) || 25,468 || 12–17(46–38) |- style="background:#bfb" | 85 || September 11 || @ Pirates || 8–0 || Steve Carlton (12–3) || Rick Rhoden (8–3) || None || 12,799 || 13–17(47–38) |- style="background:#fbb" | 86 || September 12 || @ Pirates || 2–6 || Eddie Solomon (7–4) || Dickie Noles (0–2) || Rod Scurry (5) || 11,370 || 13–18(47–39) |- style="background:#fbb" | 87 || September 13 || @ Pirates || 2–3 || Odell Jones (4–2) || Ron Reed (4–2) || Kent Tekulve () || 16,493 || 13–19(47–40) |- style="background:#bbb" | – || September 15 || @ Mets || colspan=6 | Postponed (rain); Makeup: September 16 as a traditional double-header |- style="background:#bfb" | 88 || September 16 (1) || @ Mets || 3–1 || Dick Ruthven (11–5) || Pat Zachry (7–12) || None || see 2nd game || 14–19(48–40) |- style="background:#fbb" | 89 || September 16 (2) || @ Mets || 4–5 || Mike Marshall (3–2) || Steve Carlton (12–4) || Neil Allen (17) || 4,353 || 14–20(48–41) |- style="background:#bfb" | 90 || September 17 || @ Mets || 3–2 || Dickie Noles (1–2) || Mike Scott (4–9) || Ron Reed (7) || 5,501 || 15–20(49–41) |- style="background:#fbb" | 91 || September 18 || Pirates || 6–7 || Rod Scurry