      FUNCTION BESI0 (X)
      DIMENSION BI0CS(12)
      LOGICAL FIRST
      SAVE BI0CS, NTI0, XSML, XMAX, FIRST
      DATA BI0CS( 1) /   -.0766054725 2839144951E0 /
      DATA BI0CS( 2) /   1.9273379539 93808270E0 /
      DATA BI0CS( 3) /    .2282644586 920301339E0 /
      DATA BI0CS( 4) /    .0130489146 6707290428E0 /
      DATA BI0CS( 5) /    .0004344270 9008164874E0 /
      DATA BI0CS( 6) /    .0000094226 5768600193E0 /
      DATA BI0CS( 7) /    .0000001434 0062895106E0 /
      DATA BI0CS( 8) /    .0000000016 1384906966E0 /
      DATA BI0CS( 9) /    .0000000000 1396650044E0 /
      DATA BI0CS(10) /    .0000000000 0009579451E0 /
      DATA BI0CS(11) /    .0000000000 0000053339E0 /
      DATA BI0CS(12) /    .0000000000 0000000245E0 /
      DATA FIRST /.TRUE./
C***FIRST EXECUTABLE STATEMENT  BESI0
      IF (FIRST) THEN
         NTI0 = INITS (BI0CS, 12, 0.1*R1MACH(3))
         XSML = SQRT (4.5*R1MACH(3))
         XMAX = LOG (R1MACH(2))
      ENDIF
      FIRST = .FALSE.
C
      Y = ABS(X)
      IF (Y.GT.3.0) GO TO 20
C
      BESI0 = 1.0
      IF (Y.GT.XSML) BESI0 = 2.75 + CSEVL (Y*Y/4.5-1.0, BI0CS, NTI0)
      RETURN
C
 20   IF (Y .GT. XMAX) CALL XERMSG ('SLATEC', 'BESI0',
     +   'ABS(X) SO BIG I0 OVERFLOWS', 1, 2)
C
      BESI0 = EXP(Y) * BESI0E(X)
C
      RETURN
      END
