Inclusion Criteria:
1. Subject at least six years of age.
2. Subject has confirmed diagnosis of ASD based on the DSM-V.
3. Subject with ASD whose parents/caregiver/guardian/LAR can complete informed consent process.
4. A reliable parent or caregiver who can report the side effects and communicate effectively with the research team.
5. Comorbid gastrointestinal symptoms
6. Stable medications during the two months prior to enrollment.
7. Currently receiving interventions in the community or school for ASD.
8. If female and of childbearing potential, must be willing to utilize at least one highly effective method of birth control for the duration of the study. This includes oral birth control pills, contraceptive implants, and intra-uterine devices. Diaphragms and condoms are not considered highly effective. Subjects not of reproductive potential will be exempt (e.g., post-menopausal, surgically sterilized)
9. If male, and partner is of childbearing potential, must be willing to utilize at least one highly effective method of birth control for the duration of the study. This includes oral birth control pills, contraceptive implants, and intra-uterine devices Diaphragms and condoms are not considered highly effective. Subjects not of reproductive potential will be exempt (surgically sterilized)
Exclusion Criteria:
1. Female subjects who are pregnant, nursing, or intend to become pregnant during the study period.
2. Subjects whose caregivers are unable/unwilling to cooperate with the protocol requirements.
3. Subject will be excluded who are suffering with Rett syndrome and Childhood Disintegrative Disorder.
4. Other serious co-morbid medical disorders affecting brain function and behavior, including uncontrolled seizures.
5. Subjects unable to refrain from taking non-study antibiotics for the period of the study.
6. Subjects diagnosed with cancer, except small localized basal cell carcinoma.
7. Subjects known to abuse alcohol or drugs.
8. Subjects who have undergone gastric bypass or total colectomy, or who are scheduled for these procedures.
9. Infection with HIV.
10. Infection with Hepatitis B or C.
11. Allergy to benzodiazepine.
12. Inability to stop loperamide, diphenoxylatye/atropine, or cholestyramine before the study
13. Unable to stop opiate treatment unless on a stable dose including PRN dosing, and no dose increase planned for the duration of the study
14. Known positive stool cultures for enteropathogens including, but not limited to Shigella, Salmonella, and Campylobacter within 30 days before enrollment. (These infection agents should be treated in advance of participation in this FMT study because they affect the good flora).
15. Known stool studies positive for ova and/or parasites in 30 days prior to enrollment. (Same as above).
16. Planned travel outside United States during study period.
17. Hypersensitivity to vancomycin
18. Renal insufficiency
19. Colitis
